There are plenty of HOV lane violators outside of the construction zone, and there were plenty of violators all along I-15 before the construction started. It's a long-standing problem.

There was an article on KSL a while back where a study revealed that about 1 in 3 cars in the HOV lane were violators. I have seen UHP try to enforce that lane a little, but it is almost impossible to enforce. People have figured out that the risk of getting caught is tiny, while the reward of violating that lane is big. And lots of people have decided it's worth it to drive in there solo.