One of the clear differences between federal and local attitudes is that federal police often don't mitigate their response depending on the installation. That is, you trespass in an abandoned housing area and the federal law enforcement may just go by the book, hold your group in a jail (separately and in isolation), take your cameras for evaluation and hold you until it is clear you weren't a threat - just because you were a pain in the butt when they had other things to do.
One of my 16 year old sons shoplifted a magazine (Playboy) from a Post Exchange in San Francisco. He spent 5 hours in detention in a cell (alone because of his age), had a very unpleasant hearing with the Post Provost Marshal and was discharged with a warning and could not ever go in the store again. This was before terrorism, etc.
Picture calling a lawyer and telling him/her that you're being held in a Federal facility on trespass. What is the limit on your Visa card?