Other mistake: The reporting of the horse race used to sting Doyle Lonnegan stops as soon as the fake FBI agents raid the bookmakers. J J Singleton, who is reading the results of the race, stops when he sees the raid taking place. However, that was supposed to be a live broadcast from the racetrack itself. It should have continued. Lonnegan isn't stupid - he would pick up on something like that immediately.
Other mistake: Why is the cashier sitting behind bars in the mock-up bookies when he just steps out of the side to go and fetch Gondorf? If anyone wanted to get at the cashiers they could just walk around the side where there is no door.
Chosen answer: Snyder only knew that Erie was friends with Hooker, and he wasn't on Lonnegan's payroll, just a run-of-the-mill corrupt policeman, so he didn't know Erie was involved. The Courier barely saw Erie and so may not have been able to identify him as well as he did Hooker and Luther, so Erie was unknown to Lonnegan.