Since brokers make money by trading, why restrict on IPO trading? (opens in new tab)

I see some brokerage restrict 30 days and some 15 days with Anti-Flipping Policy. If these customer do not do trading for 15-30 days, won't these brokers lose money? What do these brokers gain by restricting trading? Some authors said: They consider what they feel to be the most beneficial to their business. I am not denying above and not asking for fairness, but trying to understand, does the issuer of the IPO pay these brokers directly or indirectly? flipping :