The velocity is set a value that has been found to cause serious harm. Usually defined as "lethality", not used in the sense that it will kill you but in the sense can cause injury like blinding. The firearms folks test a variety of ways from using animal carcasses & other means to see what damage a projectile will do.
The Cansoft idea came up as a workaround for items/manufacturers not on the FRT. I'm sure if there were tests done on those they'd also fall into unregulated file but for someone who was bringing in a boatload of them the clear receiver was cheap insurance. Even if they were seized and then released the delay caused by that could sink you. When payments are due and your product is being inspected instead of on the shelf you're going out of business.
Airsoft, where nothing is hurt but feelings.