Because without ordinary people feeling that it's all right to protest; if the only people who think it IS ok are helmeted Black Flag types; then our democracy is seriously undermined.
The Serious and Organised Crime Act contains sections that effectively put the decision over what is permissable to say in a democracy in the hands of the Commissioner of the Metropolitan Police. This has led to insanity such as a group of people being threatened with arrest for having a cake with the word "Peace" iced on it in Parliament Square; of a woman being arrested, charged and convicted for reading out the names of war dead outside Downing Street. Public space outside political buildings should be protected for public claim making; they are not private spaces, and they are not public in the sense that the shopping mall is public. They are spaces needed to ensure that our leaders remember who they work for, whether they are "inconvenienced" by that or not, and it is not for the Police to decide who should and should not say what.