According to the Surveying and Mapping Law of the People's Republic of China, private surveying and mapping activities are illegal in mainland China. "Since at least 2007, it has been effectively illegal for foreigners to operate a GPS device in China."
China's state media has released a string news articles about illegal mapping efforts as a threat to national security. The article also mentions news which had been announced before, that any foreigner collecting geographical data without permission will be "severely punished".
Is it because of Taiwan?
The actual answer is "national security", and just like western "national security", it seems like it's mostly theatrical. The idea (as it was explained to me, so take this with a grain of salt) is that by introducing incompatibility and a degree of inaccuracy, you can better protect against threats that would rely on extremely detailed GPS information.
In the age of highly-detailed satellite imagery I think it's kind of irrelevant.