I'll elaborate for ya. If labor was on the credit store people would have unlimited labor which means unlimited profits which in turn creates price hikes that the common f2p player can't afford to keep up.
Granted those pots would actually be on the AH at insane prices .. but you dont need unlimited labor to have fun in this game.
With the rate at which labor is generated, in addition to the labor stipend, supply chest for new characters providing bound 1000 Worker's Compensations, and login badge rewards, and loyalty, there's really no reason to bother with having Worker's Compensation on the marketplace.
As if buying labour to make gold, is any different to buying credits to make gold ......... Its the same amount of P2W, except the labour buyers actually work for it. Its .......... exactly .......... the .......... same.
You cant get anymore P2W than direct balance purchases, its the highest thing you can sell in a game for it to be max P2W. Hell even selling BiS gear is less P2W than balance purchases.