Not true at all. Every American Airlines airplane has DC power that is provided via a cigarette-lighter socket. It works quite well with an inverter and a standard power brick, or a special DC/DC converter.
I don't use it very often because all the cords that it would require annoys me, and my laptop has 12 hours of battery life. But it is there, leading me to believe that this is not actually a problem.
(I think the fact that it's DC power limits the number of people that will use it, because you have to plan ahead, and nobody ever plans ahead for flights except seasoned travelers. If it was AC power, people would plug stuff in just for the hell of it, instead of reading a book as they do now.)
It would appear to me that most of the stupid stuff you could plug into a cigarette lighter pales in comparison to the stupid stuff that you can plug into a 120V AC. (eg. You're more likely to find a 120V AC plug on the aforementioned George Foreman grill)
Perhaps this would be (or would be posed as) a security problem, too.
I am not familiar with any specific methods of causing a large explosion that access to a 120V AC socket would allow, which not having access to one would not, with what is currently allowed to passengers through airport security [0]; But considering that producing a cigarette lighter or some nail polish remover in the cabin are grounds to be tackled by an air marshal and prosecuted, an electrical outlet is a hell of a lot of accessible power.
[0] kindly disregard my past five minutes' search history, NSA sniffer
Yep. And this is the reason that all the sockets have built-in current protection. Additionally, there are circuit breakers per seat row, which is what I discovered when I tried plugging in my 110W laptop power adapter into one of them. The other passengers on the seat row were not happy when the flight attendant said that they were not allowed to reset the breakers. Another lesson learnt - bring a lower power adapter. :(
There are DC adapter plugins in aircraft - the idea is that they are only sold for laptops and are expensive enough that few people use the.