If the storage is free you have to wonder what it really costs you.,Privacy I suspect and information that they can sell it seems to be about the money doesn't it?
That is a good question. Google for sure I wouldn't trust nor Microsoft. Not sure about Dropbox, I would think that if they did and it came to light it would shut them down since unlike Google they have no other income source ( I think). I think they rely on the ones that need more storage space and that does cost. The initial amount of storage space you get with dropbox is small and doesn't take long to fill up, kind of like our shops LOL, so either you buy more or you can get additional small amounts buy referring others and they sign up.
I really don't use Google drive and my Dropbox has nothing but manuals so I'm really not worried about them. My PCLOS is a Linux community storage, much like our Gallery is here, and I just have pictures there. The worse they could do is tell Microsoft I have pictures like this: