Because you don't enjoy being tickled, but enjoy tickling others.
The question is, do you still enjoy tickling others even if they feel like you're abusing them?
If it weren't for masochists, sadists would have no one to play with. If it weren't for sadists, masochists would have no one to play with. And just cause a sadist enjoys inflicting pain, and the masochist enjoys taking pain, doesn't mean they need to enjoy the other side of it. Hence we have the terms Top, Bottom, and Switch. Tickling is no different in that respect. We just typically use Ler instead of Top, and Lee instead of Bottom.
It should only concern you if you like tickling them regardless of whether or not they like being tickled.
Christopher