It's a phishing attack for them. They want to know if I'm drunk/high/or up to trouble(yes even though I'm sleeping) and they ask question after question to interrogate me and get me to slip up (implying they think I'm just feeding them bs about not being a criminal). They'd follow up with trick questions like 'well do you have any weapons to protect yourself?' they'd circle back and ask me multiple times why I was there trying to get me to change my story.

After a couple times of this you know their tricks and it becomes a game. I wasn't sure if it was illegal to be transient so I would always just stick with being too tired to keep driving.

Not once were questions out of sincerity or to offer help, just to find ways to pin me with a charge. It was despicable.

Many municipalities (cities, towns, and even counties) have local ordinance laws that address vagrancy or loitering, to prevent homeless and transient folks from using parking lots for a place to chill. So it's likely not legal. My own little town has a "move along" ordinance in place that says you can't hang out in a parking lot after the business that owns it has closed for the night.