I see your point. I can't say exactly why amusement parks are singled out once other places are allowing people from out of state. That's probably not such a great idea either right now, but that's my opinion that we're not quite there in terms of controlling the virus spread. The reality is, people could be bringing the virus to CA, and then doing all the things you mentioned, which is not going to be helpful. At least until we start approaching herd immunity in CA, and everyone's been able to get the full vaccine dose. That's at least a couple of months away yet.
National Parks, Beaches and even the Zoo, lend themselves more to social distancing than an amusement park, where strangers are close together in ride vehicles, in a small enclosed space (even if only for a short time), and will be walking nearby to many more people than in a big area like a National Park. And Parks and Beaches have not had hardly any restrictions for about the last 10+ months, essentially because they allow for more social distancing, although people don't always follow those rules either, when laying towels down in close proximity to others on the beach.