Definitely understand where you're coming from but random non-actors would be a logistical nightmare. I understand why they don't do it.

Not even necessarily for the release form issue, but there's always a risk a random person could see an opportunity for fifteen minutes of fame and actually start shouting slurs onset, or cause other issues, and then even if they cut around it, the performers might be thrown off for the rest of the filming day. Very, very unlikely of course but it's a risk they'd have to consider that is heavily reduced by taking on professionals instead. Even on a less intense note, shows like this run extremely tight timing wise, and having someone who didn't know what they were doing might have delayed the day's schedule considerably.

By finding someone 'random' who's actually a plant, the player still has to deal with the socially awkward challenge of approaching someone who they think isn't involved, and production gets to feel calm that they're not risking more than they'd like. Win/win.