I'm all for a persons right to protest but active duty or reserve personnel participating in a protest of any kind is wrong. Especially in uniform. When a soldiers or Marines begin to organize themselves into a questionable situations that may lead to a conflict with local or state police, maybe even national guard goes against their oath and is breaking the law according to:
http://www.sdmcp.org/Regs/Guidelines4Dissent.htm section 3.5.6. and DoD Directive 1334.1 (reference (d)
If they are just veterans and no longer subject to the UCMJ. More power to them and exercise their right to protest. But leave the uniform at home. Now you are just a tool if you protest in uniform with hippies.