The problem with this is intent.
It's one thing to do something with no knowledge it's a turn on for someone, and quite another to do it purposefully should you have that knowledge. With no knowledge it's an accidental thrill for someone, With knowledge it's being a provocateur, tease or such.
The same with the 'ethical' tickling question, only the subject of the thrill is yourself, and your 'lee' is unknowing unless you inform them.
So you get the base question: Is it ethical to get a sexual thrill from a person whom has no knowledge you are using them thusly?
People always seem to to take tickling and put it into a special box because it has a special position in social touch dynamics that crosses a few areas. Let's look at another example.
You just love running your hands through a woman's thick hair. The longer it is the better. It turns you the hell on like nothing else. One day at a office party you find yourself standing next to your co-workers wife, you all know each other, and gods! Her hair is down to her waist and is so thick, how did you never notice before? Is it cool to reach out and run your hands through it? She doesn't know it turns you on, so it must be fine right? Or how about that woman on the bus next to you, you could just say it looked so soft you wanted to see if it was. And so on...
The real issue here is your INTENT. It's what the morality of the question swings about. Not the roll of trickling in society. Not how the other person may or may not react. It's about what YOU want and want to do to get it. And is it moral to pursue that.
Usually when we want things from other people we do the courtesy of telling them we do and why.
This case is a sexual want. It probably is not appropriate to pull that out of your pack with most people outside of a select few.
And it comes down to how you view transactional issues morally. How much information does your transaction partner get? What can you take from them unknowingly and still be in the good?
Everyone comes up with a different answer. None are exactly more right or wrong, as they all come from different moral frameworks and can't really be compared that way. They can only be compared against a cultural norm.
And our culture has had a few different rules for this sort of thing over the past decades. In some it would be cool to do, in others not so much.
Myriads
We will just have to strongly disagree on almost everything you are saying.
I suppose in your estimation everyone who ever signed up for a nude art exhibit did it because they wanted to be great artists and not because they got to see a nude model? Is it ethically unfair to the model that some of the might have signed up for this particular reason, or does it just come with the territory? I would argue the risk of being sexually objectified in some way comes with the territory of interacting with other people. If you think this isn't the case, you don't understand human nature.
Can you imagine a world, where people went around asking for mundane every day things because "it turns them on" letting them know "I want this from you for xy and z reason."
Is it not simply enough to say "I'd love to give you a foot massage" or "I'd love to give you a tickle to make you laugh."
Does everyone who does anything fill out a form as to why?
"Mommy, I want this lego set, and here is my list of 50 reasons why I want it."
"Amazon here, we will say yes to this product, but only if you tell us exactly why you want it."
The mentality that would make the exception for this in the realm of a kink, is hand-in hand with a mentality that sexuality is something to be repressed and ashamed of. And ignores the fact that we are programmed to seek sexual thrills and in that regard, simply saying "i want to do X" is perfectly fine--they don't need to know the myriad (no pun intended) of reasons why you might want to--especially if all the reasons you want to might not even be clear to yourself. This road leads to eternal second-guessing and inevitable inaction--which is just a horrible way to live.
Not to mention that it is often a part of human nature to have the "unspokens" think in the same mentality of "wanna come up for coffee?" instead of asking the obvious. When a guy offers a girl a massage for example--both sides want the touching to happen, but the massage is the polite excuse to do so. Imagine getting to a massage through a series of forms indicating "i enjoy the female form and touching a female therefore i want to caress your blah blah blah" in the name of full disclosure.
In my own personal life I've had countless friends of both sexes I've tickled with. I never asked for permission (directly) usually through hints like "im gonna get you" or something to that effect. and they never asked permission with me either directly--happens in the nature of natural banter. every so often i get one whose like "wow, you tickle a lot" to which I simply reply "yeah, I like tickling." and they usually give a "i can tell!" and either run, try to tickle back or even do something to provoke the tickle. plenty of them eventually find out about my fetish. none of them are ever upset by it. if anything they are like "NOW it makes sense!" or "I knew there was something!" in fact it makes many of them more curious and want to get into it deeper.
i can say for sure that in many of the cases, if I had led with "guess what, i like tickling sexually" it would have killed much of the fun (on both sides) well in advance. if not because they would be bothered by the act itself, simply because of the awkwardness of putting it into words. sometimes less is more. and sometimes, going with the flow and presenting something in a fun and relaxed way, creates many more fun times for all.