It's "gencoins" they can't process. Which means they can't be the wallet a coin is generated to... hum, when you mine on most pools it goes to your wallet on the pool in the first instance, then it's a "normal" coin and can go to the exchange. But when you have a setup like p2pool where the coins go instantly to the wallet when the block is found, that is the sort of transaction the exchanges can't do.

I'm probably not explaining it too well, but in general, if you have to set a payout threshold or manually trigger payouts at the pool then that works fine.