Of course no one said because of sovereignty, but it actually the truth. Not all company in China are state owned. TW company has some privilege in China. moreover, mainland government limit these fishmen where and when to go fishing, there will be a fine if they not obey this. But they still do this and don't care about the punishment.
The mainland-taiwan relationship is very complex, actually some bosses or workers from taiwan work in mainland are spies. Although I don't want to admit it but it is the truth. I don't think mainland government will make it public before unifying.