First, as pointed out elsewhere, the Apple/Facebooks of the world already aren't putting their private repos on any external service (perhaps they rent iron on a cloud, but certainly not SaaS).
Second, this is the same business model as Office 365, and I'm not aware of that raising any particular eyebrows outside of the fairly limited crowd that can't trust anyone. If you're cool with entrusting your email to Microsoft, your source code is not a great leap.
No-one's going to join a company because of which front end to git they use. It's more a question "do Apple/facebook etc want Microsoft to have all of their private source code to look for exploits/rip off/hand over the government etc etc"?