I don't know why you had such a problem. There are many GSM carriers that offer SIM/pre-pay, and have for as long as I can recall.
Agreed. He doesn't say exactly when his last trip to the US was, but AT&T and T-Mobile had prepaid SIMs "a few years ago". I don't know if there are any airport shops that sell them (seems like there would be), but as you say, they're readily available in various stores outside the airport.
However, AT&T's prepaid plans suck for tourists... if you have a smartphone (and seeing that this is /., I bet OP does), AT&T will make you get a "smartphone" plan, which starts at $25 for a month of service, and doesn't actually include any data--that's an extra $5 for a measly 50MB. T-Mobile has prepaid plans that I think would work better for a short-term visitor, e.g., perhaps their $3/day unlimited plan.
But I think the best prepaid plans in the US for visitors come from "MVNO"s--basically companies that resell access to either AT&T's or T-Mobile's network, such as Airvoice or Ultra. Unfortunately, their SIMs tend not to be available in actual physical stores, which makes buying their service impractical for a visitor.