Any browser add-on could be monitoring your entire on-line activity. Why the hell does that suddenly become a problem when one's up-front about what data they collect, when they collect it, and what they do with it?
And no, they don't say "trust me". As I mentioned in my first comment, they say "if you don't trust us, unpack the add-on and check the source yourself". Which you definitely can - I just did it myself to verify, and it looks like easy reading to me.
Note: Ghostery dev here.