Yes, the window was brief for unlimited via AT&T, starting with the launch of iPad 1's 3G model. They soon stopped offering it, but customers with the plan were allowed to keep it, even if they upgraded to iPad 2.
I now switch my unlimited SIM between iPad 1 and 2, but I checked to make sure that it didn't violate my contract, because I don't want to lose unlimited. I even had AT&T note in my record that I checked. I switch the SIM if one iPad is more charged up than the other and I need to run.
Those who wanted to keep unlimited have had to keep subscribing nonstop, or lose the plan, though. It's worthwhile enough to me that I kept subscribing even while I was abroad for months and not using it.
I like the freedom of unlimited, even at the price. While in Asia, I also buy unlimited, off contract. But in Asia it's cheaper because of competition and much greater density. As someone else mention, the U.S. plans cover the country, including places with low density and rural areas. I've driven cross country multiple times and taken other long road trips, and otherwise fly to many cities, and I've been pleased with the coverage I've found, all for 30 bucks.
In Asia and Europe, there also are fewer carrier subsidized phones and tablets. You buy whatever device you want, then pick a data plan. You pay upfront for the device (say upward of $650 for a low-end iPhone 4s).