Doesn't change things. It's a liability issue and just because someone didn't sue in the past doesn't mean that they won't sue in the future.

This is different than publicly accessible federal land. In those circumstances the land is monitored for safety, closed if not, and warning signs are posted about the risk (national parks for example). This isn't that, it's a parcel of land the feds own and do not monitor, it is not safe for random people to be using it personal reasons. The people in this story never asked for nor received permission to use the land. If they want to use it maybe they should have done so before hand. There's a no trespassing sign on the property too.