From a Law Enforcement perspective (The guy who did the pepper spraying used to work for my Department) use of pepper spray against non-violent protesters is against most of the region's (northern California) use of force policies.  So most departments would not allow this and would discipline their officer.  From a legal perspective from what I gathered is that this is still considered a legal use of force based on the face that the protesters were pulling their arms away from the officers when they did try to effect an arrest. 

The main objective for these protesters is not to get their incoherent message out.  The reason why they "occupy" places (like parks and in this instance a sidewalk) is to provoke police into using force to remove them.  The protesters got exactly what they wanted in this situation.  They wanted the cops to beat, tase, or pepper spray them so that their "plight" would get more media coverage and they would be able to start some litigious action. 

Most cities/states have laws prohibiting people from camping in public without a permit, blocking public sidewalks/roads, being in public parks/on some public properties after dark, etc.  The protesters could very easily keep walking on a sidewalk (as long as they don't prohibit other lawful use of the sidewalk from non protesters) indefinitely.  They could protest until they dropped dead.  They purposely choose to "occupy" a place that has some city/state law prohibiting said action in hopes of getting into a confrontation with police.  The media does not give two shits about the protests unless the protesters are clashing with police.  If the protesters do not violate any laws then the police will not bother them.

Many of my "brothers and sisters in blue" share my opinion that this is a huge waste of time and money for our nation's police departments.  I have participated in the arrests of a few dozen of these protesters and if I had to place a label or a stereotype on the people I have arrested I would call them hippies or losers.  They are protesting out of boredom and because they are too lazy to succeed with the current system.  They want the way our government/society works to change so that they can have as much as the working class without actually working.  Again that is just my impression from speaking with them and dealing with them first hand (while arresting and transporting them).  It is a stereotype and I know that not all of the protesters are losers or hippies.  Many of them probably have jobs (although not what we would call successful careers).