Actually I used to suspect the same sort of things....maybe their accounting software doesn't allow for it or something. Except for one thing.....nobody from HTC ever says that. All I get is a "we don't allow special characters" which to me implies they just disallow them for no particular reason.
HT once told me that they'd "never" allow such names as mine. That suggests that they don't want them, instead of not being able to have them. Of course, I'm persistent and realize that in the online world "never" seldom means never, so I continue lobbying them. And I will continue lobbying until such a time as either they let me have "J_A_B", a better product comes along that makes me forget AH ever existed, or AH dies.
Honestly, I suspect the problem is that they didn't think to make the game accept such characters when they programmed it (none of the HTC crew's ID's required such characters), and aren't going to change things to make ONE person happy.
J_A_B