Really? but arent these international waters? what is the status of these waters?
You, like many non-Chinese, consciously or subconsciously, assume Taiwan is an independent country and its relationship with mainland China is that between two countries. That is totally wrong. Taiwan and mainland China, for practical and principled reasons, are in a state of unfinished civil war, dating back to the late 1940's. This is by far the most accurate description. Therefore, there is no border between the two, temporary or permanent. There is not even a temporary peace agreement, there has been truce, on and off. In other words, war and conflict can break anytime.
Before the late 1990's, Taiwan had air superiority over mainland; its military aircraft regularly flew over the so-called mid-line over the Taiwan Strait, simply because it could. Mainland China military aircraft, on the other hand, had to fly close to the shore along the mainland because it was at disadvantage relative to Taiwan air force. In 1999, the then Taiwan president Lee Tenghui declared that the cross-strait relationship is "special country-to-country relationship." That statement triggered a crisis, PLA aircraft started to fly across the so-called mid-line to the side of Taiwan. From then on, the two sides implicitly observe the mid-line during normal time. It might surprise many people, Taiwan's ADIZ (Air Defense Identification Zone) during the '50s and '60s, established by the US, cut deep inside mainland. Taiwan military aircraft, including U2, regularly flew into mainland for intelligence gathering. Similarly, upto until the 90's, mainland warships regularly avoided crossing Taiwan Strait due to the superiority of Taiwan Navy.
These days, you see some foreigners, particularly westerners, often indignantly blame China "threats" democratic Taiwan. They speak like China is acting like a bully to a democratic, peace-loving independent nation. Well, democratic or not, it's irrelevant to the nature of the relationship; independent it is certainly not.
Both ROC's and PRC's constitutions stipulate that both sides belong to one China. Furthermore, PRC has anti-secession law specially for Taiwan, it stipulates several scenarios in which mainland China will use all necessary means to reunify Taiwan, including non-peaceful ones.
The US supported the nationalist government during China's civil war in late 1940's and "lost China." The US and PRC established formal diplomatic relationship only in 1979, seven years after Nixon visited China, thirty years after ROC government exiled to Taiwan. The primary obstacle for the normalization of Sino-US relationship is Taiwan problem. In the end, the two countries established diplomatic relationship only after the US agreed to cut diplomatic recognition of, abolish mutual defense treaty with, and withdraw military from Taiwan (断交、废约、撤军 ). After Korea War, the couple of times China and the US almost came to blow were all because of Taiwan (1958, 1996).
I'm writing this lengthy note to remind people that China will go to war for Taiwan should its sovereignty in Taiwan is under threat, either because Taiwan goes for de jure independence or enter into some kind of military relationship with a foreign actor. A lot of people in the west don't understand the nature and history of Taiwan's relationship with mainland China, and try to stir the pot just to spite on China's face. It's very dangerous.