We host our SDK and you remotely load it at runtime. The benefit is that we detect breakages due to gmail and automatically update the SDK. Your users just need to refresh gmail to load your extension and the latest SDK which is compatible with any changes Gmail makes.
In general though, we treat the SDK as its own product and have several competitors to Streak using it. We have millions of end users using apps built on the SDK from major companies (Dropbox, Hubspot, etc) so we're pretty careful with our terms.
The main goal is to retain the ability to maintain compatibility between different extensions being run by the same user and being able to respond quickly to changes in Gmail.
By the way, there's a real opportunity to own the content side of gmail development. Articles on getting up and running, like building a basic Rapportive clone, would be incredible.
EDIT: we have automated systems that help with this though making our turnaround time incredibly fast. We also see the changes to gmail before 99% of users so usually our updates are out before users even refresh to get the new version of gmail