"This will prevent skirmishes from happening. The cops won't have any excuse to start shit, and neither will the protesters."
In the words of Kevin Spacey in Superman Returns, "WROOOOONG!"
1. Many of the protestors, especially the bandana- masked Anarchist vermin, go to such events specifically to start fights, especially with cops.
2. Considering that the "Security staff" will undoubtedly consist of fellow protestors, They would pretty much undoubtedly allow the protestors through to attack the cops.
3. If the "Security staff" attempt to block the police and/ or prevent them from doing their jobs, they themselves could be arrested for aiding and abetting, obstruction of justice, possibly assaulting an officer of the law, and God knows how many other charges.