Did they also agree to give up their native language and the right to practice their religion?
Who says they have given up their native language? Perhaps if more if them spoke Chinese, they wouldn’t be left behind in the economic development.
The right to practice their religion is not a right at all. The Tibetans do not recognize the separation of Church and State. Few Americans this generation remember but this separation exists primarily not to protect secular political life from the manipulations of religion, but the inverse. Influence is a two way street, if religion dictates your government, then your government can also dictate your religion. Cuis regio, eius religio.