It can still be assault and kidnapping when the police do it, if they don't have a good reason and follow the rules. Same applies to surveillance. The question is not cops vs. non-cops: it's what the valid reasons and rules for the cops are.
It is still assault and kidnapping, just legally sanctioned, and for some reason we feel compelled to call it an "arrest". Just because its LEO performing the assault doesn't make it any less of an assault.