Just to think about the blocking of installing apps from unverified source on Android by Google, isn't this an exclusive contracts upon users?

The ruling is not that Google can never have exclusive contracts for anything. If they rent Moscone Center for an event, there's no requirement that others be allowed to use the same space at the same time.

And there is no exclusive contract between Google and users with regards to sources of apps. It's a change in technical requirements for the platform.

In law, the actual thing matters; just being able to draw vague parallels doesn't mean anything.