This amazing internet thing has opened that hobby up immensely, just like it has for everything else. We slingshot geeks ("catapult" geeks in the UK--cute, huh?

Well, I went to order a new frame from a custom manufacturer and discovered that many manufacturers won't ship slingshots or even slingshot supplies to Salt Lake County. Evidently, it's illegal on some level. I don't know if it is illegal to OWN the stuff (I can buy rubber, leather, marbles, buckshot, ball bearings, etc. all over town...geesh!), and I want to find out what the laws are. Y'all on this forum are really good at finding specific laws. I've come up dry; any ideas?
Of course, if it turns out I'm a criminal for flipping steel balls through a soda can with a forked stick and some rubberbands while archery, paintball, air guns, and darts are all fine, I am going to find somebody to gripe to about it. I truly can't imagine the logic, and I could simply ship it to my dad in the next county, but why my hobby and why only in this valley? Geesh.
I'd appreciate any direction anyone could give me in finding out what the law actually states, as I don't want to lose my priveleges by getting caught participating in my lifelong hobby, ya know?