There's too much wiggle room there, and they know it. The words "for a legitimate purpose" are unnecessary and leave prosecution wide up for discretion. Either you make a certain instrument illegal or you don't. You don't muddy the waters by adding superfluous language. You trample rights when you subject an innocent person to being arrested for having a 2" blade or whatever. "Sir, we're arresting you so we can run an investigation to make sure you were carrying for a legitimate purpose."

If you want to criminalize the behavior, then you criminalize the behavior, instead attaching such language to a law about a tool. For example, you make stabbing someone with a knife illegal outside of self-defense. It's a subtle but hugely important difference.

The problem here is that the government (including law enforcement) wants the easy way out. They can't (or are too lazy to) enforce the laws that are ALREADY THERE. These are feel good laws that do absolutely nothing but restrict the rights of law abiding citizens.