Edit: oops, I think I did it again. After re-reading what you posted, it seems like you are saying that after a sentence is served (a sentence is not limited to prison), a person regains the constitutional right to free movement, so why not regain all other constitutional rights as well? In other words, why take away the right to vote OR the right to bear arms if physical liberty is reinstated?
Good point. I have no explanation why, in practice, physical liberty is given greater protection than other constitutional rights. Still, my original point that there is no constitutional justification for giving the right to vote greater protection than other rights remains valid.