It would be interesting to add the currency rewards the attackers would gain from the hashing process. I bet the cost may be greatly reduced, and may even make the operation profitable, without much illegality...
Of course, if you try to double-spend as part of the attack: it's fraud!
The court system isn't run by idiots who will just shrug their shoulders because you thought you found a loophole.
They would instead laugh at your "clever" loophole and throw you in a cell.
Your legal defense has to convince a judge and jury. You are not going to be able to do that.
You'd make way more money on some kind of double-spend attack. If you've got 51% for sure you would probably hit up multiple exchanges at the same time to magnify your reward.
If you control more than half you can dictate terms, you can fight back against the other miners, and in a protracted battle you will ultimately prevail. With 49% they can always eventually override you.
If someone else has a block in those last 6, it would mean that someone else (who I assume is an honest node) saw your block. At this point, your entire chain will be published and likely overtake the mainline (otherwise, the honest miner wouldn't have bothered with it). Now, it appears to the merchant that your transaction has not yet made it onto the chain, so you have to wait for another 6 blocks confirming the transaction.
The only way I see around this is if you can partition the network. However, not only is this difficult, if you can partition the network, then you no longer require 51% to do an attack. In fact, if you can assure that the target is on the smaller partition, you require 0 mining power to do the attack.
EDIT: Essentially, at 51% you can make a "rule" that all blocks must be yours.