Quote from: TierNolan on February 29, 2016, 10:38:16 AM

This problem isn't really specific to currency trades. An attacker could buy lots of stuff with an altcoin and then roll back all the payments a week later.

There are some egregious distinctions.

Attacker identifies his own UXTO which the community can then decide to blacklist with a checkpointed fork. Thus taking away the attackers income and causing the attack to be a loss.
Attacker will have a very difficult time purchasing things at sufficient scale that doesn't identify him in the real world. Whereas stealing balances will be impossible to prove for cut & choose
The victims can prove they were double-spent by long-range chain reorganization. This isn't an absolute proof of an attack, but community evidence gathering at any sufficient scale of attack should come to a consensus about the existence of an attack.
There is no way for any user to opt-out of this attack. Whereas, users can decide to not use a DE which exhibits being a risk, and/or to not use a block chain which enables such scripting. Thus in contrast, the community's diligence against this attack in existential. Meaning that the DE (and the general form of scripting that enables it) is likely to be shunned by users/investors once (in theory) the attacks occur. Whereas, a 51% attack is likely to be dealt with by community action by increasing hashrate and/or checkpointing.