I still say part of this is all the people getting in for free. The kind of people who wouldn't go to a park if they had to pay, but "hey, it's free" are often the kind that are rude in other aspects of their lives. The trash problem seems to be mostly in California. I note places like Zion are closing down more because of snow/road maintenance issues and the like.
The National Forests here are free, with very limited refuse disposal, etc. and I have never really seen ridiculous trash problems like those recently pointed out at National Parks during this shut down. Of course usually the National Forest areas are a bit harder to get to, and I guess they draw a different class of people.