One reason China wants to keep American signals (electronic) reconnaissance aircraft (the U.S. Navy EP-3 and the U.S. Air Force RC-135) away from their coast is because of the apparent weaknesses in the Chinese air defense systems. The Chinese air surveillance radar system and the anti-aircraft forces its supports are, in reality, a ramshackle collection of new and elderly equipment that never quite meshed into a single reliable system. That means that carefully analyzing these network from international waters (at least 22 kilometers from the coast) reveals vulnerabilities that attacking aircraft could exploit. This is doubly troubling to the Chinese because the Americans are known to share this kind of information with their allies, especially Taiwan, South Korea and Japan.
Aside from that very real vulnerability, as China develops more powerful electronic devices they have become determined to keep the details secret (so their electronic wonders will not be easily defeated in wartime.) To that end, they have requested that the United States eliminate the use of aircraft and ships to monitor activity along the Chinese coast. Most of this is electronic surveillance. Some is simply keeping an eye on what the Chinese are building (in the way of military facilities along their coast.) The U.S. recon operations take place in international air and sea space, but the Chinese know that this does not stop the Americans from picking up lots of useful information. China has implied that if the U.S. does not cease this snooping, there will be more confrontations with Chinese aircraft and ships. Similar confrontations took place with Russia during the early years of the Cold War. Dozens of American military personnel were killed. China is also trying, without success so far, to unilaterally modify international law, to extend its territorial waters (that other nations cannot enter).
The EP-3 and RC-135 are getting old and will eventually be replaced by UAVs but for the moment they are the best tool the U.S. has for sniffing out things like gaps or vulnerabilities in a potential enemies radar coverage or air defense systems. The main reason for all that popularity is that the EP-3s and RC-135s are very good at what they do and have been much in demand since September 11, 2001. The RC-135 in particular is a flying vacuum cleaner of electronic signals. Built on the same airframe as the KC-135 tanker and Boeing 707 airliner, it carries two dozen people to operate all the electronic gear. Exactly what kind of electronic signals the RC-135 can pick up is classified but apparently includes any electronic device the enemy in Afghanistan is using and most of what China has. With the withdrawal of American forces from Iraq and Afghanistan more U.S. electronic recon aircraft are available for duty along the Chinese coast.
In Afghanistan RC-135s gained a lot of valuable experience against an armed enemy. The RC-135s collected a wide variety of electronic signals in an area and analyzed them quickly. The analysis effort is looking for patterns. The bomb teams leave signs electronically (cell phones, walkie-talkies) or visually (images captured on surveillance cameras). Using the right math and analytical tools (software and computers) you can quickly discover where the bad guys are coming from and have the ground troops promptly raid the location.
This kind of work is popular with the RC-135 crews because they are getting a chance to do in a combat zone what they have long trained for. Moreover, it's relatively risk free, as the aircraft fly beyond the range of machine-gun or shoulder fired anti-aircraft missiles. In addition, the most productive work is done during night missions, when the bad guys can't even see the RC-135's high above. Off the China coast there is actually more risk as China has demonstrated a willingness to use their own interceptors to try and block American recon aircraft from operating in international waters.
The most blatant example of this occurred during early 2001 when Chinese fighters regularly harassed U.S. reconnaissance aircraft off the coast of China. The recon flights over international waters had been going on since the end of World War II. Then on April 1st 2001, a Chinese J-8 twin engine fighter clipped a U.S. EP-3 four engine turboprop aircraft. The J-8 crashed and the pilot ejected (but was never found.) The EP-3 limped away and made an emergency landing at a Chinese military airfield on nearby Hainan Island.
Early in the Cold War, American recon aircraft were often attacked by the Russians and several were lost. But by the 1960s things had calmed down and in 1972, the U.S. and Russia signed an agreement to cover encounters at sea. Russian and Chinese electronic reconnaissance ships still cruise off American coats. Since 2001 China has been regularly violating that agreement.
Buzzing foreign recon aircraft is a no-no. International conventions stipulate that the smaller, faster aircraft must stay clear of larger slower planes. That doesn't prevent you from intercepting and escorting aircraft flying off your coasts outside your territory. What apparently happened between the J-8 and the EP-3 was a second rate pilot (the Chinese now have more modern Russian aircraft for their best pilots) coming up behind the EP-3 and finding out too late about how troublesome the air turbulence larger aircraft leave behind them can be. The J-8 is generally recognized as a pig of an aircraft and hard to control. The Chinese put themselves in an embarrassing position with the aviation community by accusing the EP-3 of causing the accident. Aside from the physical impossibility of that, EP-3 pilots are selected for their calm demeanor, not suicidal tendencies. Moreover, the EP-3 pilot’s primary task is to fly the aircraft along a precisely defined route (to get the best reception and to make sure the aircraft stays in international air space), not play games with fighter jets.
After the April incident China backed off on such close encounters but now the aggressive attitude has returned. Not only in the air but also on the surface as China has formally claimed control of the entire South China Sea and is trying to force everyone out (unless they have Chinese permission to be there).