First, DRM are not necessarily against all FOSS licenses (see GPLv3 discussions). And then, there are a lot of FOSS applications on Android...
http://www.zdnet.com/blog/open-source/no-gpl-apps-for-apples...
I was talking about GPL specifically. Signing a binary is a modification that goes outside the realm of GPL.
First, DRM are not necessarily against all FOSS licenses (see GPLv3 discussions). And then, there are a lot of FOSS applications on Android...