It's the derivative works part of the T&Cs for GDrive that makes it a little wonky, I don't want to give them full license to create any derivative work but I'm happy with them making changes for technical architecture.
But that's so selective and does miss the part where they say your content and copyright is all yours, so it justifies picking on Google for being supposedly creepy.
As others have mentioned, it probably is about producing thumbnails of images or something like that.
>You give us the permissions we need to do those things solely to provide the Services.
This give Dropbox the ability to modify your images to create thumbnails as well if that's a part of their service. It also allows them to do whatever else they want without being explicit and hiding behind vague wording.
But that's so selective and does miss the part where they say your content and copyright is all yours, so it justifies picking on Google for being supposedly creepy.