I will give it a shot. The FDA has a regulation that governs lasers. The laser in a peq-() is a class III laser, because it is infrared. Class III lasers can cause damage to, or blindness of the eye, hence the FDA regulation. The Regulation basically says you have to have all infrared lasers contained so that the beam will not come into contact with any ones eyes. Uncle Sam was able to get a waver to this rule for its aiming devices. However the waver also says that they can not be sold to individuals, new or used.
There however is no regulation governing a person building one, or modifying existing red, green or blue units by changing out the laser diode, as long as he does not sell it.
The above is my understanding of the regulation, but you may want to read it for your self as things change from time to time.