It's not against the law to point a squirt gun at a cop at night, either. Do you think that person bears no responsibility for what happens when they do?
Actually it is.
Carrying a condom full of flour doesn't mimic a situation in which a cop's life is in danger, so it's an apples-and-oranges comparison. This situation with the flour could have been settled within a few hours if justice were being served. That it was not is the responsibility of the law enforcement agency.