Why are paid visitors required to ask each docent that they photograph for permission to put their picture on social media? The CA State Park website encourages the public to photograph events for publicity.

The docents sign a photo release to volunteer there. The parks stay open because of visitors, and get free publicity through social media. As long as they are in a public place, filming and posting on social media is encouraged.

Yes

It's seems like a common courtesy these days. The requirement is not legally binding.