I don't know the answer but agree with "bicker"... it's a liability thing, I think!
I can tell you though that when my son was under 2, we still purchased him a seat (this was on Jetblue)... he was so unhappy about being strapped in that they let me hold him during takeoff and landing (again, since he was under 2). We were using his regular seat but he was just very unhappy about the whole thing.
I have submitted the question to Delta/Song and Jerblue about what is required as far as seats go... b/c we are travelling again and he's over two now. There are new 5-pt harness seats for travelling but some airlines do now allow them. These seats are much smaller and are carried easily.
Good luck!