Because harlequin shrimp (please dont get me wrong here, I've not researched them much because I have no desire to keep them) but dont they NEED live food? At any rate - I dont commonly see foods specifically labeled towards one particular fish (occasionally I'll see "tang food" which is just minced up nori). But just because a specific name branded food doesnt exist for a specific animal it doesnt mean that it should not be kept. Do you find people who feed their turtles the "feeder" goldfish unethical? If not, what's the difference? Just the price is all I can see. Actually the feeders are treated much worse than any ornamental fish/invert that I've seen. So in my mind, its probably much "nicer" to feed a starfish that's been well taken care of and not suffered its entire life. I do agree that linkas shouldnt be in the hobby but thats not in the scope of this dicussion.
And with your last point - that's why I dont keep snakes
And it makes your point even more arbitrary. I think that if someone is willing to look into and keep harlequin shrimp properly it is one of the more ethical things anyone in this hobby is doing. Sure, plenty of this hobby - if not all of it - is immoral and unethical, however in the case of harlequin shrimp, I rarely see a post that says "help my harlequin shrimp is dieing, what do I do". Maybe that's because they are rarer in the hobby than I imagine (I've never seen one in person) or for whatever reason.