At some point the merchandise and people has to cross their fences too, and can be caught there. I do not understand the fuss, unless they have a Tardis in the middle of the reservation.
The agreement allows the indians to pass through the fence at designated gates, gates not manned and are simply there for convenience tor the indians when they cross. Sometimes the indians jump the fence to avoid walking to a gate, and that gets them a stern talking to from the BP. . .but that is it.