Every document stored in iCloud (music, tv shows, movies, contacts, apps, books,pages documents, etc.) are downloadable. If you couldn't, it would make for a shitty cloud (you can upload all you want, but you can never access your files? How does that even make any sense).
You can also store all of Apple's iCloud documents in other services. Gmail contacts, Amazon Music Locker, Dropbox, etc. I'm not sure you are aware of how iCloud works from the user's point of view.
As for logins, that's not what is being discussed. No one makes a fuss that you can't log into your gmail account using your own server's PAM configuration, or on your NT domain. No one complains that you have to sign into Netflix to watch your Netflix content. You're right that Cromebooks require a (somewhat ironic) Google account, and that it's something to consider. But even that lets you use iCloud, Evernote, Dropbox, whatever. That's just for logging in to your computer and (of course) to get ever more people using Google services.
> Now it's true that you could argue that people who buy into these ecosystems know what they are getting into, but that's nonetheless 5 examples of devices specifically tied to back end cloud services, with the easy potential for more devices as other vendors attempt to get into the Android/iPhone/Nook/Kindle/ChromeBook space themselves.
Except that wasn't my question. Where is this a problem? How is your data locked away? Even with the Chromebook, if you decide to buy an iPad instead, it's simple enough to transfer all your data into iCloud, of if you prefer, just keep using all the Google apps as is.
I'm definitely not saying there aren't downsides, or things to consider, just that having your data locked away isn't one of them.