The reason law enforcement/medical/entertainer units can't merge is because merging messes up the properties the units provide - or at least, that was the reason given when the ability to merge was removed from these, way back when. (Interestingly, the mechanic line still has the ability to merge, despite being subject to the same argument and despite my pointing it out repeatedly over the years. It means those are the only civilian garrison units which can merge, for now - even the animal healer line is not allowed to do so.) Incidentally, law enforcement being unable to merge was the reason given for criminals not being allowed to merge either - otherwise merged criminals would become far too powerful for anything which could see them to defeat.
Personally, I handle criminals by stationing multiple law enforcement units in each city - at least one set to investigate, at least one set to arrest any detected criminals, and at least one set to suppress crime. As my needs get higher, I add on units as needed - it rarely goes above five or six units in a city, but it keeps things under control.
Personally, I handle criminals by stationing multiple law enforcement units in each city - at least one set to investigate, at least one set to arrest any detected criminals, and at least one set to suppress crime. As my needs get higher, I add on units as needed - it rarely goes above five or six units in a city, but it keeps things under control.