Unless the online database at the State Legislature is out of date, the DPS does not have the authority to make a stop solely for a seat belt violation.
Title 28, Section 909C.
All I can tell you is that there has been a huge radio blitz about it, here in Az. The speaker says something to the effect of "I'm giving you this friendly warning, because the cops aren't writing warnings. So click it or ticket". Make of it what you will.