This wasn't a stupid decision at all, OP is just misunderstanding the purpose of the music.

The goal isn't to prevent videos getting uploaded of cops standing around thinking about ham sandwiches

it's to prevent videos getting uploaded of cops abusing their power and brutally attacking people

and if that had happened, the video would've gone viral anyway. We've all seen them. Streisand Effect only occurs when a video would have otherwise gone unnoticed-- that wouldn't have been the case if the thing the cop was trying to cover up had happened.

It's not like it's the first time they've done this, either. It's just the first time it's gotten this much attention. This going viral wasn't inevitable, it was just happenstance for hitting on a slow news cycle.

And I don't know why him admitting to what he was doing is relevant at all. It's not like admitting it breaks a law; it was evident what they were doing every time they did it. It's not like some prosecutor was waiting around saying "If only we had an admission of guilt, we can put an end to this practice forever!"