They don't care. The purpose wasn't to charge them, since the powers-that-be already know that protesting isn't a crime.
The purpose was to get them off the scene. It doesn't matter that the charges are automatically dropped, what matters is that they drag the protesters away in that moment.
Why isn't the very act of disturbing their protest by violence unconstitutional? (Edit: unless the protesters are infringing somebody's right to property ofc)
Dropped charges don’t necessarily mean that the records have been cleared. These 57 demonstrators may have an arrest on record for the rest of their lives. They’ll have to explain that while trying to rent a house, get a job, or any other number of things.