Not banned I don't think, Honda bought the AMA a few years ago and had some rules changed... like no lead in the fuel, displacement changes and wieght requirements.
They're not banned, they just aren't competitive against the modern 4 strokes anymore, due to rule changes. So basically none of the big teams use them anymore. If you really wanted to you could bring a 2 stroke to a National (or Supercross) event and attempt to qualify, people are actually doing that right now. Look at the qualifying results from Indy Supercross this year:
Scroll down to the number 37 qualifier (Alex Ray) and look at what bike he has, it's a KX250.
http://results.amasupercross.com/data/2011/10/sxpcovr.pdf
Here's the one from the last Outdoors race (Freestone), check out the 67th qualifer (Tanner Young), he's on a YZ250.
http://americanmotocrossresults.com/data/2011/02/450_qc_ovr.pdf
The movement I've seen was to change the rules to allow the 2 strokes to have bigger displacement, as that's about the only way [I know of] in making them more competitive.
However if you go look at the amateur ranks you'll notice that they do have equal displacement rules, which means 250F's race against 250 2 strokes. However the 250's still aren't very competitive, especially in the lower classes, however that has to due with the lack of skill. It's most likely fed by the mentality that a bigger bike makes you faster, however as you probably well know, for the great majority of people that isn't necessarily true.
There now you have my .02 cents.
Edit: Guess I should've read the above link first.
The petition
is for the AMA to change the displacement rules.
This petition was generated to enact the following change to the AMA Pro Racing Rulebook Section 3.1: Allow 100-150cc 2-strokes in the 250 class and 151-300cc 2-strokes in the 450 class.
The Pro AMA Rulebook currently says this:
3.1 Engine Displacementsa. 250 Motocross Class: 0 – 125cc 2-Stroke / 150 – 250cc 4-stroke
b. 450 Motocross Class: 150 – 250cc 2-Stroke / 251 – 450cc 4-stroke