The first is, does Tibet have the right to self-determination and when it was a theocratic state did it have that right vis-a-vis its supposedly more progressive neighbor China? Would it have had that right vis-a-vis its supposedly more progressive neighbor India had that nation chosen to invade it?
I think the answer is unreservedly "yes." (All who wish to defend China's claim to Tibet on the basis of old maps, or the assertion of sovreignty over Tibet by the emperors may raise their hands in objection at this point.)
The second is, why do otherwise apolitical New-Agey types defend Tibet? I think it's because of the mythology of Buddhist pacifism (ask Sri Lanka's Hindu minority for guidance on this one). If you're into meditation, ringing bells, etc., of course you solidarize with those who you believe to share those values.
For what it's worth, many of the most active supporters of Tibet are also strong opponents of U.S. militarism. Our local Buddhist Peace Fellowship here in the San Francisco Bay Area spends much more time talking about the government's bomb-design lab out in Livermore than they do talking about Tibet.