They probably figure that the "What I can't drive in the HOV lane anymore?!" excuse is worth at least one warning. I also am not sure there is any enforcement. In my 2 years here I've not seen a single person pulled over for what appeared to be an HOV lane violation.
With how dark tint is allowed in AZ, I suspect at least half the vehicles in the HOV lane are a single person.
And since I'm bitching about AZ HOV lanes, I'll also add that its insane to me they allow commercial vehicles (other than buses) and vehicles with trailers in the HOV lane. /end-rant