Petzl (a company manufacturing outdoors equipment) has the following statement on their website:
Self-belaying is prohibited!
We cannot ignore the fact that some people use the GRIGRI as a self-belay device. Many internet sites give tips on modifying your device for this use.
Above all, this technique increases the risks to the climber. Firstly, the climber does not hold the brake side of the rope.
Additionally, in case of a fall ,the GRIGRI can be blocked against the rock, the positioning system (positioning strap), or other, thus negating its braking function. The climber could fall to the ground. Finally, remember that any product modification outside of Petzl facilities is formally prohibited (see Instructions for Use).
Self-belaying with the GRIGRI is prohibited.
Are these provisions actually enforceable, in accordance to US law? If it matters, Petzl products are sold in regular stores that don't require customers to sign an explicit waiver before making a purchase.