I don't think that's an entirely fair assessment of what they've done. Most of their core apps are developed in house, or by a different group that I'm reasonably certain they fund (though I could be wrong on that point).

Additionally, Ubuntu does the exact same thing, and has been doing it for longer than Elementary. As soon as you click "download" on their overview page, you're sent to a contribute page. Sure, Ubuntu has a link in plain text that directly says you can download without contributing, but it's not immediately apparent either.

Arguing that it's immoral for Elementary to ask for money, even if they don't fund Ubuntu, Debian, and the Linux Kernel directly is the same as arguing it's immoral for Canonical to ask for money because they either don't, or only very minimally fund Debian and the Linux Kernel from what I've seen.