I think it's okay. School's understand that you may have other places to choose from - it's why they ask for a deposit in the first place. Similar to renting an apartment, a deposit guarantees that if you flee (go to another school), the school gets some "damages."
In any case, enrolling a class is not completely black and white. It's why there's usually a waitlist of sorts. Do you find that unethical? Using your given logic, I would think that it's unethical to build a waitlist and even more so to not tell the waitlisted candidates where they are ranked.
I'd say it's great that you have such a strong moral conscience but like in business, sometimes you have to look out for #1. If the school granted your deferral and you are paying a deposit, I think you can relinquish any remaining guilt. If you choose to attend another school, well it was your choice to part with the money in the first place.
Men want to run the world, women want to run Louis Vuitton.
Part of the reason I'm going to HEC Paris?