All the major retail brokers sell their order flow, so you generally eat this cost no matter who you use. In addition, the hidden cost is fractions of a penny on the dollar (so still better than the $5-7 a regular broker charges per trade anyways).
They get a good deal on the commission and they get enough money from the gold subscribers to cover the commissions across all users.
A fix subscription package is user friendly and easy to understand. It can be "better" for business than a variable commission on trades.
Plus, I bet they are banking on the existing brokerages not responding until things are too late for them. Most brokerages offer free trades and other perks, but limited to "high-value" customers. They won't drop trading fees until they absolutely have to, but by that time, their low-value customers will all be gone.