It makes perfect sense if you don't think about it too much, which I suspect you are. Here's the general pattern:

You don't have to do X to help Y, even though you said you would, and it'd be nice. But nobody would blame you if you didn't do X.

Indignant, you make a big deal about how you don't have to do X and it's Y's fault for not taking care of things earlier. Unsurprisingly you look like a selfish shithead in the eyes of decent people.