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.

One of the weaknesses of an altcoin is that it has less security. 

The exchange creates a large market of things that you can (irreversibly and quickly) buy with the altcoins.  The underlying issue is that the altcoin has low security.

The concept behind the POW system is that a single POW can cover lots of trades.  This breaks down if enough people get together to try to double spend.  They can split the cost of double spending over all the attackers.