It does seem a little odd, but as a resident of one of those states I think it's awesome. I can peruse the lot with zero chance of being approached by a salesperson. IMHO it actually facilitates car sales, because one day a week people can look at new cars without immediately feeling like prey.
You can do that online any time. What you see in the lot on Sunday is the same as what you would see online.
But as a resident of one of those states as well, I hate it. Not because I want to buy a car on Sunday but because if the dealership is open the service section of the dealership would also be open.
No, what you see online is a digital representation of a physical product. What you see on the lot is an actual physical product that can be touched and smelled.
Once upon a time, there was a used car dealer in Austin that unlocked all their cars when they opened in the morning. And the only time a salesguy would ask "Can I help you?" (anyone hate "What can I put you in?" too?) was when they were walking by you after talking to someone else. And they took "I'm still looking right now" for an answer.
I felt really bad when I couldn't find something I liked there.