"Are there people who carry canes just for self defense, even if they don't need one to walk?"
Sure. I actually needed to use one for a couple of years, until I got both hip joints replaced. In the process, I acquired a few of them (canes). So, it's not too far of a stretch to grab one now when out taking a walk in the local neighborhoods. Besides, it's kind of fun to have something to play around with while walking. Who cares what other people think, that see you walking briskly while carrying one. They don't know if you have a trick knee, or something like that. It offers security that you wouldn't have otherwise. I like my Irish Blackthorn for most of my casual strolls, since it doesn't really look much like a "cane" as a crook cane does. But in the end, it's your business as to why you have it with you. It's not illegal.