I understand why the police can't shoot them down with firearms, but spent air rifle pellets are not particularly dangerous.

That’s not entirely true, which is why you can’t shoot them into the air with impunity. From an risk analysis POV it’s highly unlikely that a shot fired into the air will actually hit someone, but the amount of energy the projectile has if it does hit someone can only be calculated with lots of complex maths

^{1} and a shed load of assumption. Who it hits, and where it hits them will also be a major element in the outcome.

1. Ok, not that complex, but lots of assumptions if it’s to be calculated in advance.