That's a fair point, and perhaps I was overly harsh (I'm somewhat sleep deprived, so sorry if I did!).
But I think the distinguishing factor is contriving things. There's a difference between "did you get a new pedicure" while people are discussing their beauty regimes and "did you get a new pedicure" when talking to your boss about the quarterly returns. It's the difference between questions where "this might turn me on" is a factor vs ones where it's the only factor, and I think a lot of "lines to get people to admit they're ticklish" fall into the latter category - it's clear that you're only asking this because you want to get off on the answer and it wouldn't be something you would otherwise say, and if nothing else people can usually pick up on that (we've all met the guy who keeps asking about women's foot care routines in great detail and clearly isn't fooling anyone).
I guess it's the same as any other ulterior motive. Think the difference between me inviting you over while hoping you bring your cute friend I have a crush on, vs me only inviting you over because you'll bring your cute friend I have a crush on. One is pretty normal and you'd probably laugh off if you found out, one is manipulative and likely to upset you if you find out. I think that's the meaningful distinction here. Asking someone about their ticklishness only because you would find the answer sexually arousing is probably, on the scheming side, no matter how subtle you are about it.