As many of the posters above have noted the issue here is in the definition of masochism. While, yes, then enjoyment of pain is a major trait of the identity, it can also include the concept of distress, on the physical, mental and emotional levels also (Enjoying Humiliation if a form of masochism).
Tickling can indeed inflict distress across the three aspects (Physical, Mental, Emotional) and thus it falls firmly into masochism.
Myriads