IBs can take their cuts from commission or from spreads. Either way, it's money going into their pocket every time a trade is closed.
The downside to a service (EA, Managed, Signals) being your IB and putting trades into your account is this:
They are getting money, win or lose. Naturally, if they make money, you will stay with them longer. On the other hand, if they start to lose money and lose clients, 2 temptations crop up.
1. Modify the system to try to fix it - and desperate modifications frequently make things worse, not better.
2. As long as things are going down anyway, trade like crazy to rack up those payments before the clients pull the plug.
Even if a service like this is profitable, there's always the temptation to "split" a trade. Instead of going for 50 pips, go for 25, close the trade if successful, and open another trade to go for the remaining 25. Twice the trades = twice the payment to the service provider. For clients, it's 2 sets of spreads/commissions to pay.
Deliberate over-trading is called churning. Wildly churning an account is easy to see. Doing a little extra trading isn't so easy. There's not a clear cut line on where normal trading ends and churning begins.
Paying the service that trades for you on a per-trade basis is placing the temptation to churn your account in front of them. Personally, I recommend against putting that temptation out there.