I just recently took a trip and was in conversation with a woman at the boarding gate about this very question. She had a double carryon suitcase and was told she would have to claim it in baggage b/c the plane was too small to accomodate her luggage. She was not happy and I didn't blame her.
She said she understood that she could carry on two items and it did not seem fair that she could not do that. But, she would not have been forced to jam her clothes into two small carryons if she hadn't been so determined to avoid the baggage fees.
I think it is ridiculous, but the claim is that the airlines have to regain financial expenditures to compensate for the loss in fuel costs.