Why take the chance when you're about to have a lot of IPO cash? Go with Google for a few years until you (hopefully) become FB. Right now they'll focus on features, signing advertisers and users.
I imagine they had a very strong bargaining position willing to commit this much, for this long. Even if Snapchat wasn't a valuable brand for Google to brag about, this amounts to ~10% of the yearly earnings [1] of Google's cloud business (SaaS and IaaS, of which I suspect SaaS like Apps for Work is the lion's share)
I'm sure they're getting a good deal, and can focus on features and getting their platform profitable, as you said.
1. "..at that pace, Google’s cloud could generate $4.1 billion in revenue in 2016" http://www.networkworld.com/article/3029164/cloud-computing/...