Resonance on a bridge is only able to become an issue if you have everybody actually marching in step (and it’s worth noting that these days they make sure bridges can handle way more than they’re expected to). Getting a crowd of protesters on a bridge walking together is just gonna have the weight of the protesters.

Demanding that businesses not poison people downwind/downstream from the factory is not giving an unfair advantage to businesses that have good processes.

Yes, they saved the businesses having to train new employees to replace the dead ones.