I'm surprised nobody has mentioned the most obvious (and innocuous) motivation: to expose young developers to google's cloud platform so that they'll be comfortable with it when they become big-time CTOs in a few years.
The network effects for cloud services are nowhere near as strong as they were for operating systems (end users don't care which underlying platform is used) but there is an advantage to having more developers familiar with your service.
This makes the most sense to me as motivation for this kind of give-away. Both familiarity to soon-to-be-promoted techs, as you said, and establish themselves as the system to migrate AWAY FROM.
To have the first $100k of server time's code written on their platform is not meaningless. Perhaps I'm a sub-par wizard, but host migrations are a very real pain point in my experience. With $100K's worth of time spent developing on Google's platform, that pain would be pretty well developed by the time a startup would get around to worrying about it.
The network effects for cloud services are nowhere near as strong as they were for operating systems (end users don't care which underlying platform is used) but there is an advantage to having more developers familiar with your service.