That's fine for cops (although then you've still got the problem that the legal system is heavily stacked against the poor), but I disagree that all unions are

That's fine for cops (although then you've still got the problem that the legal system is heavily stacked against the poor), but I disagree that all unions are inherently bad (even for cops, the union could just be limited to salary and benefit and general work environment negotiations). If we're going to make all unions illegal, then we better start enforcing some pretty strong anti-collusion laws. Let's ban trade and industry associations for starters. It is an established fact that companies will collude against their workers, allowing workers to collude against companies does nothing but level the playing field.