Is it ethical to force graduate applicants to decide before they hear back from all schools?
The April 15 Resolution technically applies only to offers of financial support, not to offers of admission. There’s no clear-cut rule saying a school can’t require an early answer for admissions, saying it’s unethical to do so is a difficult claim to make. To my mind, it’s definitely poor form and suggests that the program is either (a) quite prestigious and doesn’t need to worry too much about yield or (b) rather noncompetitive and desperate for enrollees, but they’re risking rejections as well with the hardline approach they’re taking. (Funny how extreme cases in different directions can take the same approach!)

However, given that the program in question is unfunded, it's completely unreasonable for them to expect you to commit irrevocably to an offer of admission, particularly since they are effectively not committing any additional resources to offer you admission. I would thus feel no moral qualms whatsoever in withdrawing my acceptance if another, funded program were to make an offer before the April 15 deadline.