There are a few national parks and monuments that cost nothing to get in, but you will take a boat tour, walking tour, or similar to see what's what. You can't use your annual pass to pay for those, so I suspect that's why the tours cost and getting in does not.
I am also enjoying your reports. I know they take time, but those of us who can't be on the road right now find that this is the next-best-thing.
Donna