The reason you don't see many is because it's a huge hassle. There is a lot of paperwork, the bikes have to fully comply with USA specs (known to be among the toughest in the world) etc. There are also plenty of fees along the way.
I don't know how the ones that do get in manage, but suspect they aren't done completely legally, unless as you suspect, there is a military advantage.
Bikes over 25 years old, no sweat - anything can come in, no compliance required, other than usual lights horn mirror etc.
Note that the different states have different regs, and some are more lenient than others. Worth checking with your local motor vehicle office what the local regs are.
There was an article a while ago in a magazine about this - the conclusion was it worked out to be next to impossible, not worth while except for something truly special.
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Grant Johnson
Share the Dream!
at:
www.HorizonsUnlimited.com