It was a rider on a bill called The Credit Card Act of 2009. It is way down at the bottom called "sec. 512". IIRC It was some politician's pet project when he had a problem with being banned from carry on National Park land in southern Texas (Think it was Big Bend NP) where there was a bunch of drug smuggling going on. Understandable. It makes the National Park unit adopt the state laws that it is located in with firearms carry. Some parks like Delaware water Gap have two sets of laws depending on if you are in the PA or NJ part of the park so it is not perfect legislation.I am not sure this is true. I seem to remember a suit that made Federal parks allow for concealed carry on their property, which is what I THINK lead MD political clowns to make our state parks Gun Free Zones.
I have no idea about the legislative history of the MD state park ban.