Probably because they don't really want people to use it. I imagine the license has to be fairly open to allow for unencumbered distribution with android, but it's a part of the android brand. Every time it gets used outside of android makes it a little more generic and dilutes the android brand a little bit.
If it were visually distinguishable from Helvetica, maybe. But it's really not. It's more modern, more complete, and probably "better" in some typographical way that I don't understand. But it's really not. What this represents to the free software community is a repackaging of a Helvetica-like font in a way that doesn't suck. To anyone concerned about the "Android brand" it's a wash.
So it's ok to use it in personal or commercial projects (by including a copy of the license an propper attribution).
The font can be found in the folder data/fonts on the Android SDK 4.0 / r14, and also the Readme file that states the license.