I find it a bit suspect that the officers did not secure FG with the seatbelt because of a concern for their own safety but had no trouble taking him out of the van and putting him in leg shackles.
Here's a generic pic of a Baltimore police van with someone in the back: http://www.haaretz.com/polopoly_fs/..._gen/derivatives/landscape_640/2354652940.jpg There is a narrow right side and a narrow left side separated by a solid metal partition, leaving very, very little floor space imo. It seems to me an officer would have difficulty getting into the van with an arrestee, so would stay outside and help the arrestee onto the seat. Looks like seatbelting someone would involve the officer stretching his arm across the chest of the seated person, who could easily bend down and bite the arm, even while shackled. Just guessing though; maybe there's a way to do it without risking infection. Getting him out would be easy if he was seated or prone on the floor ... and not belted in.