As the answers suggest, there's a difference between encrypting everything with the same key, and encrypting between two parties.

Arguably, encrypting all snapchat traffic with the exact same key is almost worthless, and they could just send everything unencrypted. If a snooper knows they're looking at snapchat traffic, decrypting it is trivial.

At least with a key exchange, you have a good chance that only the targeted recipient can view the photo.