Snapchat "takes a picture" by just grabbing a frame from the viewfinder instead of, you know, using the proper API.
This could have made sense back when Android imaging situation was more of a mess, but the current API obviates the need for this.
It's likely just a case of "it it ain't broken, there's no budget"
It is pretty broken though, since the image looks terrible because it skips a lot of the image processing steps.
It definitely made sense because Snapchat is only used for ephemeral photos, and taking a "proper" photo is extremely slow on many Android phones (or at least it was until recently).
That said they should have just implemented both solutions and let the user pick.