I don't think it's as black-and-white as that, since sometimes people are also not as black-and-white with their expectations. If there's an actual agreement that the item is on hold (which I think should only be done if you are more or less certain to buy, but that's a whole 'nother can o' worms), then I agree it would be unethical for the SA to sell it to another person. If it's more of a "I'll have a look next week", then a phone call to the loyal customer to confirm before selling it to another is common courtesy. If it's a "Meh, sure I'll see it", while the walk-in is positively epileptic with happiness with finding the item, then I think she should have it. It's a case-to-case basis, and you just have to trust the SA to balance fairness and common sense, whether you're the loyal customer or the walk-in customer.
You've never asked an SA if they had something in another color or size? Just curious.
And yes, I've had my fill of mental exercise in this thread. Sorry for derailing the topic so ratchetly!!!! Love and peace, ratchet ones. When other discussions come up, let me know if you want to be a character in a hypothetical scenario.