There are many very, very political people inside Mozilla. Some of them may even want to commit political violence. Political violence seems to be a problem that just grows and grows, so how can we be sure that it's not supported in Mozilla. These would be a very small minority of Mozilla of course, but the problem is that you don't know who it is. And it only takes a single extremist to betray your users. To get your users injured or even killed.

The same concern will of course apply to any other data harvesters, but that's for another thread