Re: no such thing as "free" storage
You aren't familiar with how these file synchronization services work, are you? They aren't like network shares that you can mount as pseudo-local filesystem locations. Be it Dropbox, Google Drive, Microsoft SkyDrive, Ubuntu One, or whatever, you have to install a daemon (not a device driver) to run on top of the operating system, usually in user space. They all generally monitor a specified directory's contents and synchronize changes in either direction when they happen.
With regards to installing a proprietary piece of software that does things without your knowledge, Google Drive's APIs are actually quite open, so there's nothing stopping you from writing your own synchronization software for it. In fact, that's exactly what has been done with Linux so far. I can think of at least two clients for Google Drive available for Linux that are not made by Google, one of which is proprietary (Insync) and one of which is open source (grive).
There just simply isn't a standard device model for synchronization across various operating systems. Even local filesystems are implemented differently throughout the different operating systems. For instance, NTFS, a proprietary file system, is supported only in user space in Linux, apparently due to licensing issues. Ext4 and btrfs on the other hand, both open source file systems, aren't supported in Windows at all.
Regarding privacy and security, I will grant you that you do indeed have a point, but that is a risk you run with any remote storage solution, free or not. In fact, all the services I named above do actually provide additional remote storage capacity for nominal fees.