It's their revenge on you for cutting them. But seriously, it's meant to deter things from eating them. Which of course is one reason we (as humans) eat them, because humans are weird and we sometimes like pain. Kinda like hot peppers, they got spicy to discourage anyone but birds (who can't taste spiciness, and are the best seed spreaders) from eating them. But that kinda backfired in the case of us weirdo humans.