That doesn’t sound bad to me? Email attachments and file uploads have worked like that on the web way before instagram.
The issue with the Instagram model is that it looks like you are just doing things locally on your phone, and haven't uploaded yet. If you have second thoughts and decide not to post the pic, Instagram could keep the photo.
That said, I don't really see why they would do this. What would it gain them to keep millions of discarded photos?
My point is that it is feature clearly people appreciated, not a dark UX pattern.