Some people perceive flick knives as out of bounds; this is common in some countries, including much of the US.
When I first saw them commonly for sale in Italy in open-air markets, it seemed a bit foreign. In some places they seem to be widely accepted -- and even seen almost as toys, or at least as not a big deal.
In other places, though, they are seen as weapons carried by people who are dangerous; and they are associated with the criminal element, or with people who do not mean well or have bad intentions or tendencies.
So the police may have reacted to the flick-knife images or associations in their culture, which might be different from the way they are perceived by you. Even the words -- flick knife and switchblade -- carry different connotations. There seems to be quite a bit of variation in the way they are perceived in different cultures and by different individuals.
Please don't let it affect your view of all police. Some of them are good people who are there to help, and not all of them would have handled it this way.