I vaguely recall, SoCal, that this 18hrs thing was done here before, and it is contained within the
primary U.S. Visa legislation. I can probably dig out some references you could follow up.
The reason why the TSA etc don't refer to it is because they don't create law; they are just writing their own interpretations of it because they like to feel they can.
The bottom line is however purely practical - I doubt any school (except maybe some very small ones) in the USA will touch the candidate with a 20ft bargepole unless he comes with the gold plated paperwork...
My experience of people involved in the FAA training business (including some in the UK) is that most are simply not interested in reading the regs. Not just the security regs but also stuff like eligibility of previous training. I spent 2005 trying to do the IR here in the UK but gave up after just about everybody claimed that all eligible previous training had to be done with FAA instructors
These are the real problems in this game.