Did everyone miss the 'short chase' part of the article? After that, I'm sure the leo had cause to check the gun. Just my opinion, of course, but MOST times people doing nothing wrong don't get a lot of hassle from police-types. Most likely the cop checked the gun for his own safety (not knowing why these guys didn't pull over immediately) and then found the extra rounds.
I'm not saying the law isn't b.s., but there's more to this story than the number of rounds in the magazine.