They are supposed to attach to the bumper. I have seen them before being used by repo companies to haul cars back. They drop the bumper cover, drill some holes in the bumper to put the "universal brackets" on, and hook up. They put the fascia in the car being pulled and when they get to where they are going, they take off the brackets and put the fascia back on. The only way anyone would ever know it was done is if they pulled the fascia and found the 8 holes in the bumper.
I spoke with a U-Haul guy who told me that U-Haul used to sell them and that the hardest part about them was figuring out how to attach the brackets.
Anyway, here's a link to an example:
http://goo.gl/m0NzeRuss