It's actually a huge dispute whether or not it's incorporation is legitimate according to international law. It certainly has it's own territory, ethnicity, culture, and government in exile.
Google's policy seems consistent to me: actual territorial disputes between sovereign nations are handled one way, and separatist movements are handled another. What would make Tibet special?
I'm not sure what you mean by this. There's a very real territorial dispute between China and India right now, which are both sovereign nations. The area within the dotted lines is under dispute, as to who it belongs to. It's not clear that the default answer is India.
The usual criteria I've seen mapmakers use is the reality on the ground. If you want to travel there right now, who will you need to get a visa from? At the moment the answer is India, so it is part of India.
Whether that is the way it should be or will be in the future is another kettle of fish. Whether that answer will make people upset is also a good question. It looks like Google is taking advantage of the Internet to try to tell everyone what they want to hear.
But right now if you want to take an international flight there you'll need a visa from India. So it is part of India.
I don't doubt the dispute about whether it should be a part of India or China is real. But currently it is a part of India. Maybe China has legitimate claims. Maybe it doesn't. That's what I took your "should" to mean.
What's different about Tibet? China claims Tibet is a legitimate part of its territory, but others claim that China is an illegal occupier. However, at least for now, Tibet is a part of China.
http://en.wikipedia.org/wiki/Tibetan_sovereignty_debate