I once did some consulting work for a seat manufacturer, and here's what I learned: the FAA mandates that seat manufacturers perform crash tests on the seats they make for each airline. The tests are quite rigorous and, to make a long story short, the manufacturers have to smash apart a lot of perfectly functional seats in order to meet FAA safety guidelines.

The FAA doesn't care whether passengers recline during takeoff or not. BUT if the airline is going to allow passengers to recline during takeoff, they would also have to perform additonal crash-tests with the seats in the reclined position, thereby doubling the testing expense.