This was an episode that showed the limits of their budget and/ or imaginations. It's amazing that we can see CGI Gorillas fighting on network television. But that seems to be the extent of their idea. So after the setup of assistance of the Police force they have no involvement in the finale.
I suppose it could be that they didn't want to risk the police being mind-controlled as hostages again, but it would've helped if they'd said so. And why couldn't the speedsters be mind-controlled?
I think it's a waste of Grodd to have him stage a physical attack like this. I've said before, he should be the behind-the-scenes Big Bad for an entire season, an unseen manipulator using his mind control to influence people and events, to take over from the shadows, and only gradually be revealed to the audience and the heroes as the mastermind. It'd be a nice change from Yet Another Evil Speedster.