Ephemerality doesn't truly exist without secrecy and privacy in this case. It's one reason I don't use snapchat, because the "experience" is misleading.
It does, in fact, exist. The experience of ephemerality is all that's necessary, and the experience is that you take a photo, it exists for 10 seconds for all intents and purposes users care about, and then disappears. Disappears here means that I, my friends, and people I care about ever see it again. They don't, I don't, so I'm happy.
Remember, you are not their target market. Their target market is 2% people like you (nerds concerned about privacy and indirect/intangible principles) and 98% people who just want the experience.
I still think those 98% would be upset to know that someone somewhere still has access to those photos. It would certainly change the behavior of a huge chunk of them w.r.t. the service.
Remember, you are not their target market. Their target market is 2% people like you (nerds concerned about privacy and indirect/intangible principles) and 98% people who just want the experience.