That's a strange sort of "choice": if I don't buy things at the store, I starve to death. If I don't buy land (or rant it, or inherit it), I die from exposure.

Like I said above, a mandatory vote does not require individuals to make a political statement any more than taxation does, unless it is coupled with a limited selection of candidates. We can talk about the propriety of a mandatory vote, but the legality of one in the United States seems fairly cut-and-dry.