The marchers are penned into a limited space, and if something were to go wrong the stampede effect might be bad.
My family and I marched today with a lot of folks, and except for one instance where a cop asked us about our respect for authority because we marchers kept breaking a fence joint (she got a lot of laughs), there was no tension. We were walking, the police were protecting, and we hope everyone else was watching.
That wasn’t the case on May 4, 1970. Though everyone ended up watching. Four dead in Ohio.