These people you speak of are not protesters. They are rioters. The United States Constitution guarantees the right for people to peacefully assemble. They are being violent and someone will end up getting hurt. If you want to protest that's fine, but don't get violent. Police officers have a job to do and they can't sacrifice everyone elses health to allow others to be violent.
If it is the strong protest. We have to use the violence protest to stop the protest of people. But if it just a little bit protest then we don't use it. For example : some country have a crivil war and it is a strong protest so the police or etc. that thy protect something or somewhere that very impotant. They have to use it to protect them and that places as well.