Reply #47: Presidents (and other executives) do it all the time.

[Unvanguard]

Obama has himself: he has made enforcing drug laws against medical-marijuana users a low priority, he has directed immigration enforcement to focus on strengthening border security and deporting criminals rather than long-term residents (who, technically, are just as illegal), he stopped the deportation of non-citizen widows in advance of Congress fixing the problem.

By political tradition, executive power has become something that allows for real discretion. This is not necessarily a good development, in the general case, but it is a real one.

Edit: I agree that it is ambiguous territory, which I why I said "arguably" in my original post, and for both legal and political reasons I don't think his failure to do it is indicative of some kind of opposition to the project of repealing Don't Ask Don't Tell. I am merely seeking to point out that this demand is not coming out of nowhere, and is on firmer ground than your OP suggests.