What good am I that I did nothing to stop any of it?

Those cop cars could have been about anything given the information you had at the time. When you found out, it was already over. That's not your fault!

Is there anyone you can tell besides us that this happened, preferably someone local or who works with homeless people?

When I got back, there was yellow tape everywhere and all the tents were in a pile waiting to be loaded into a garbage truck.

So they weren't even allowed to gather their belongings before they were kicked out, and then said belongings were destroyed? I'm not a lawyer, but considering the obsession with protecting property in capitalist "justice" systems, that's where I might start with getting some compensation for the displaced people if I was one. Fucking scumbag cops.