Isn't it the purpose of GPL to enable the user to exercise his freedoms, specifically, the freedom to make modified versions of a program? If the user is not given a way to install his own (modified) versions of the program, or if he is no longer allowed to (!), he has no way of exercising his freedom to run modified versions of the program.
Deeming jailbreaking/rooting illegal (unless it is done with permission from the vendor) kinda makes the whole thing incompatible with GPL if you ask me.
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