Do you mean they offer to pay for the limit orders on the second - slightly marked up plan right? Anyways, who would buy the marked up plan?Quote from jplazard:
MB has two commission type plans one is commission based which the spreads are tight, the other is called an EXN type plan where the spread is marked up slightly and you just pay spread.
They do offer on the first plan pay for all limit orders going in and out of the market(buy/sell limits and take profit going out) so if your that type of trader you would not pay commissions at all. I still trade forex with them strictly for the pricing the platforms are not that great though.