It already is very illegal, the problem is the guys calling in are good enough to not be caught. Especially with VoIP software, VPN's, TOR, proxies, etc. it's simple to make an untraceable 911 call using stuff like Skype.
Most SWATTING happens with online gaming. I used to be a gamer (still am but not as much as I used to) so I always made sure I kept my gaming persona separate from the real me, meaning it would take a lot of work for someone to connect my gaming username to the real me. That's the biggest way to stop SWATTING, if they don't know who you are or where you live they can't call the cops on you.
As for the 7-Eleven story, I wonder if the guy who called it in got in trouble. That's hardly anonymous like the normal gaming situations are.