On the one hand, this seems objectively bonkers, but on the other hand, I can kind of see how it'd make sense. If everybody using Snap uses a phone, and all the phones have more or less similar resolutions, and Snap doesn't allow any sort of zooming, then really the pixels on the original screen are exactly what you need, so why bother taking a new, different picture?
I'm pretty sure I can answer that question (because using a camera as a camera gets you a better picture, because some other phones that will view your photo will have higher resolution, etc), but I do appreciate a clever hack. I'd love to know the actual reasoning behind this. Maybe there was some weird permissions issue between displaying what the camera sees and "taking photos" that affected some users at some point?