Hey, if you want to be angry at Snapchat, be my guest. I have never used it so I don't know for sure what they claim. I do assume that they never explicitly say your DMs can't be read by them, because lawyers exist.
There is no victim here. The student teacher did say she was going to shoot up a school. There were no charges filed, and she rightfully lost her job for poor judgement.
If she wanted a truly private conversation, there are plenty of options. Hell, even Facebook encrypts DMs by default, iirc.
People need to learn the difference between "feels private" and "is private".