To me, the ridiculousness of the scene was that what appeared to be a SWAT team reacted--within a ridiculously short time --to a security guard's emergency call that their cameras weren't working at the local city archives, and that they spotted someone who may or may not have been the vigilante (given they only glimpsed him for a second). A team of heavily armed and armored officers charged to engage a single person invading a file room, and, though no hostile action had been taken at them that I can recall, began spraying burst fire--through a shelf-crowded room full of paper--at one the intruder with little warning.
I think that the SWAT team's reaction was to the presence of Arrow rather than to the theft of files. The police want "the vigilante", bad. And know that he is very hard to handle.