It's probably has to do with liability issue. There is a chance the OEM screw of the pedal will be too short to accomondate the extra thickness of the strap. Also there is no way of telling what is the condition of the OEM threads. It can be badly tapped and slip through QC. If the screw strip out and personal injuries resulted, the manufactor of Power Grip may have to admit flaws in their installation of their products. Safer for them to provide proper hardware.
On the other hand, if you the end user decide to mount it differently, then the fault will not be theirs.