That's really clever - I like that. However, couldn't you argue that if your enemy attacks you, you may not need quite as big a blockade to accomplish your objective? In that case withdrawing with a % move could be better, right? For example, in the game you posted, if you had blockaded the turn before, I think a percentage blockade might have been better than the transfer only one since his stack in Mexico was smaller and you didn't have to worry about him depleting your stack too much.
Turn 4, Arrow838 does not know how many armies will attack Panama at the end of the turn. He wants to blockade, and he needs some armies to make a decent blockade -- if he has leftovers at the end of the turn, he wants those leftovers to transfer to Mexico and be part of the blockade. So first, he uses a transfer to (Mexico) to "mark" the armies for the blockade in case the opponent attacks.
I know many people are joking here, but in fact I believe this blockade to be a very smart move. People tend to only look at what happens in SA and CA, noone cares about long term. Long term the guy secured West US, which is a great +5 bonus and he only gave up a +3. Well done indeed.