@Roussette
Protests can rapidly get out of hand. Especially at the minute when a lot of protestors seem to be leaning towards violence. Members of the crowd seeing people protesting ina designated area could encourage them to strike up there own protest where they stand. Although according to reports, they weren't actually in the designated area.
This is not the occasion to protest. With so many children and families around and so many tightly pack crowds, if unrest does break out, the chances of people ending up with serious injuries or worse is very high.
Plus if someone did decide to lob something, there is absolutely no guarantee the police would catch it before it could harm anyone. And while that person would be arrested, pandemonium would instantly break out.