I noticed this blog post that speculates that a 51% attacker could be prosecuted under fincen rules. If that were true wouldn't that also mean that all miners would also be subject to the fincen rules (even though they don't have 51%)?
I think it is a moot point, essentially.
The reason a 51% attacker would do that attack is to either harm bitcoin (and thus has no intention of selling newly mined coins) or to gain economically by double spending transactions the attacker previously made (i.e., to where the attacker obtained value which was paid for prior to the attack, but then was able to double spend as part of the attack and thus defraud the seller who accepted bitcoin payment from the attacker.)