Enjoy your milkshakes, man! :)
I think "the people" are choosing to reward you as a witness, in part, because of the great things you've done to build Cheetah. We could go in circles talking about "needs" versus "wants," and you are technically correct, but I do think it provides great value which should be appreciated. People can't pay for something if they don't even know how much it costs. If they did know how much it costs to run Cheetah, they might appreciate it that much more.
I think anyone is free to profit off anything which the market demands and doesn't violate anyone's negative rights (i.e. go against the non-aggression principle). That's the beauty of open-market systems. When people pay for things with their own money, they more accurately understand the value of those things. Cheetah might be undervalued. It might be something people don't appreciate enough which makes them think spam, plagiarism, abuse, identity theft, etc is not an issue they need to care about. I think it's an issue we all have to care about because this is our collaborative commons. Maybe Cheetah could be wildly profitable for you (once people properly value it) which would bring in cheetah competitors, innovation, competition, etc and might even make your job easier while improving the entire ecosystem. Reminds me of how they paradoxically started saving endangered species by creating a market for selling hunting tags.
I'm totally fine with publishing what I do with my witness salary (at this point) because I haven't done much with it. I pay for expenses only, for the most part. I keep it powered up and use that delegation to help projects I think are adding value. That said, I did recently (I think it was last week?) trade out some STEEM (I think 1000?) for my own personal cryptocurrency trading. I hadn't done that before and it felt a bit like your milkshake. :) Ultimately, I think the community pays witnesses to do a job with the funds they are given and they are free to use those funds however they want as long as they are directly paying people who vote for them (though with the recent community witness popping up, that line may get a little blurry). The way those funds are used will be part of how they are evaluated as witnesses. I'm all for transparency.