I never really thought of it as a fetish, but I can definitely see the appeal of it. At the risk of sounding a bit weird, I'll admit that my tickle and bondage fetish is partially derived from this concept. In my late teens, I often had to babysit my younger sister and her friends and I often found myself the victim of their mischievous plots, often being tickled purely in a playful and teasing manner.
I was occasionally tricked into playing games with them and found myself bound and gagged a time or two at the hands of these brats. Now let me emphasize something, this was NOT a sexual experience as it was just playful encounters. It did however, provide a weird thrill for me, kind of a rush if you will, wherein I realized that I enjoyed the concept of being made helpless and teased by others. I also found the power exchange to be interesting and the idea that I was supposed to be in charge, but was suddenly helpless and in a way, relieved of my duties was for some reason fascinating.
It wasn't until my last two years in college where I had a chance to experiment sexually with guys I dated (and a few girls as well) that I realized tickling could be a sexual thing for me, and the teasing that goes along with that was something of an accessory.
I've concluded that while these experiences with my sister and her friends that occurred during a formative time for me (late teens early 20's) the rush playful rush I enjoyed from such games, carried over into my sex life as an adult. I hope that doesn't come across as creepy, because it really all was very innocent.