I think it's perfectly reasonable to not support all distros, *if*
1. At least one significant distro is supported completely, i.e., it works there (this is then a base to start from), and
2. Google cooperates with other distros in getting it to work with them as well, if/when those distros are serious about doing so, and
3. No code/build scripts/etc. are made in a way that would make them hard to run on other distros.
We already have (1), and I hope that (2) and (3) as well. If so, then I find no fault here.
Now, if this *wasn't* open source, then I would have very different criteria. In particular, I would expect at least the major distros to be supported by the vendor. (Of course I prefer if it's open source, but I'm just contrasting. For open source, I expect less from the vendor because we the community can and should do part of the work.)