The whole appeal of Snapchat is that nothing is permanent, so you don't have to really think about what you're sending.
There would be obvious commercial advantage in doing this, just for starters.
Data mining services like facebook gives you a solid picture of the interests and connections that the target person broadcasts publicaly. This is useful information, and it is exploited by any number of parties ... and people are (hopefully) aware of that.
Data mining snapchat gives a deeper picture of a person's interests and connections, as they are broadcast whilst that person believes that this information is entirely private.
Aggregating data from both sources (which would be almost trivial), would provide a very detailed view of the target. Much more than the sum of each dataset alone.
And it matters not whether the owners of Snapchat are completely and utterly above-board with everything .... there is the question of who owns and operates the network that all this info travels over.
Bottom line is that all of that "ephemeral" data that is going through snapchat is no more private than facebook, or youtube, or HAM radio, or telephone calls, or email, or SMS, or pretty much anything electronic.
There is no real privacy with this, but its easy to provide the illusion of privacy.
You can't, but that's not really the point. The point is that it's not permanent in a form that other users can access. Sure, maybe Snapchat stores every picture I ever sent. That sucks and should stop. But at least Grandma can't go back and look at some 2 year old picture I took with some buddies.
Truthfully, I don't really care if my Snapchat data is being mined. I'm not looking for absolute privacy. I'm not sending sensitive or private information. The ephemeral nature of the medium keeps me from worrying if a picture is unflattering or could be construed as in bad taste out of context.
It's like wondering why someone who still thinks good and bad humors mediate health isn't worried about their cholesterol, there's literally not the machinery available to make that a concern. This is one of the biggest reasons that we should teach coding in schools. Even if you never write a line of code in anger, knowing vaguely what's going on at least gives you a chance to be a consenting participant in technologically mediated arrangements.