Why is it illegal to say bomb on a plane?

Because of the irrational fears of terrorism in present day Western-society. Although the chance of actually becoming the victim of a terrorist attack in Western nations is so slim, it's virtually neglible, due to the widespread and sensationalist portrayal by the media. It has now gotten to the point that just the suspicion of anything terrorism related can set of a wave of panic that could have fatal consequences at worst and injuries, lawsuits or big fines at best.

If you would shout "bomb" on an airplane on purpose, any consequences that may arise from it will be at your cost (literally!), such as the costs of diverting an airplane mid-air because of a false bomb threat.