I don't want this to be a fight, but if you can find in the CFR for vehicle lighting equipment (
https://www.law.cornell.edu/cfr/text/49/571.108) where is says modifying stock headlights and/or aftermarket HID's are illegal, I will concede to your statement.
I do believe, after I Googled as your suggested, that you found an NHTSA "request for comments" regarding 49CFR571 -- and that has to do with glare from OEM and aftermarket HID's. I do believe there is a requirement to have some kind of glare-limiting cut-off now (haven't found it yet) but that in and of itself doesn't make modifications for HID's illegal. The excessive glare is illegal (and it is in Montana though no one has quantified it yet).
All of the internet sites I found referred to the same NHTSA document, which is not law. So there were a LOT of people giving bad advice on this topic. I'm presenting this so that you and other forum members have actual facts, rather than internet opinion.
PS: Thanks to [MENTION=103267]Saker[/MENTION], now I'm thinking of doing HID low beams again! I'll probably keep changing my mind back and forth for the next six months or so.