Had they arrested Gabby (or Brian), sure, it may have changed the course of events. But, there's no guarantee it would have. And they couldn't hold either one of them for more than just one night.
The Moab PD recognized domestic abuse, but the circumstances (and injuries) didn't rise to the level of arrests.
Maybe if Gabby would have said she was scared of Brian, but she didn't. Instead, she took responsibility, and her story matched Brian's story.
I get the feeling that van life was so stressful, as long as they pursued it, they were at risk.
But that trip was Gabby's dream, and it seems as though she was determined to see it through even though their relationship was souring.
If Gabby couldn't forsee Brian's growing tendency to violence, and neither of their parents picked up on it during the visit home or phone calls, I can't imagine LE could have foreseen it, especially because the Moab stop was so long before the murder.
I think a lot of departments deal with mild cases of domestic abuse and have to make judgment calls as to whether they should take stronger steps to intercede. On the one hand, they want to prevent injury, but on the other hand, they don't want to punish the participants if it's a one-off event.
Just MOO
I think the Moab PD could be more tactful in their statements, but I don't think the Petitos have a case they can win.