Libertarians generally subscribe to several maxims. Here are two: 1) That everyone has the right to express their views, providing their expressions are within the law.
2) That it’s a good thing when people feel the effects of the social costs they impose on others.
A recent debate on Radio 2 – about street demonstrators possibly having to pay to protest after police look to refuse to close roads for them to demonstrate – pits these two maxims against each other. However misjudged I find the views of groups like "The Campaign Against Climate Change", "The People’s Assembly Against Austerity", "Global Justice Now", and "Friends of the Earth", there is clearly a case here of the immovable object of people's freedom to protest coming smack bang against the irresistible force of valuable resources being exhausted by the presence of these demonstrators on our streets, in this case in the shape of exhausted police resources, blocked roads, temporary traffic regulations and potential disturbances of the peace.
On the one hand I suggest libertarians wouldn’t want a situation where people’s right and ability to protest was predicated on their ability to pay. But on the other I would prefer protesters to feel the costs of their actions. So while I am all for defending people's prerogative in being able to demonstrate against actions they dislike - when those actions come at a social cost to others, some are arguing that it is not particularly unreasonable that they should pick up the bill for these negative externalities imposed on others.
It may not be unreasonable, but I don't think it should be desired either. The state chooses to have a police presence at a demonstration (it is compulsory to notify the police of any planned demonstration), not the demonstrators. A very risk averse state apparatus operates under the ethos that modern life is safe but expensive. Any restrictions of this kind on the ability of people to protest legally is bound to restrict freedom of expression, and at the same time increase the likelihood of people protesting illegally - and that is not likely to make organised demonstrations any cheaper or safer.