The tactical evolution that evolved relies on two military tactics that are thousands of years old- the tactical superiority of light infantry over heavy infantry, and the tactical superiority of the retreat over the advance.
Heavy infantry is a group of soldiers marching in a column or a phalanx that are armed with weaponry for hand to hand, close quarters combat. Heavy infantry function as a unit, not individual soldiers. Their operational strength is dependent upon maintaining the integrity of that unit. Riot police are heavy infantry. They will always form a line and advance as a unit.
Light infantry are armed with ranged weapons for assault from a distance. Light infantry operate as individuals that are free to roam at a distance and fire upon the opposition with ranged weapons. Cops firing tear gas, rubber bullets, water cannons, bean bag rounds, etc. are light infantry. They remain to the rear of the phalanx of riot cops (heavy infantry) and depend upon the riot cops maintaining a secure front and flanks to provide them a secure area of operations.
Protesters function fluidly as either light or heavy infantry. Their mass, because it is lacking in organization, functions as a phalanx, having no flanks or rear. Lack of organization gives that mass the option of moving in whichever direction it feels like, at any given time. If protesters all move to the right, the entire group and supporting officers has to shift to that flank. While the protesters can retreat quickly, the police can only advance as fast as their light infantry, supporting staff can follow and maintain a secure rear (if the mass of protesters were to run to the next block over and quickly loop around to the rear of the riot cops, the organization of the cops would be reduced to chaos). If that police cannot assemble with a front to oppose protesters, they are useless. The integrity of that tactic is compromised, and unable to maintain internal organization, the cops revert to individuals engaging in acts of brutality, which eventually winds up on the evening news and they lose the battle regardless of whether they clear the park or not.
Because of the lack of organization in a crowd of protesters, light infantry cops firing tear gas, etc. has little effect because it just serves to disorganize a group that relies upon disorganization in the first place. All it really does is disorganize the riot cops, who then resort to brutality.
The lack of weaponry on the part of the protesters grants them the luxury of opposing riot cops at close quarters, or remaining at long range in a refusal to engage the heavy infantry riot police at all. They have the advantage of the retreat, they can quickly move away, or in any direction, and the heavy infantry riot cops lack the swiftness to respond.
So far, all the occupations have, in a grave tactical error, agreed to engage the riot cops when they march in to clear parks. This has been a show of bravado that has the tactical benefits of providing media coverage of the brutal methods of police and the benefit of draining the resources of the oppressor by forcing them to incur the expense of arresting and prosecuting people for trivial offenses. Fist tap Dale.