That implication is exactly what Colin provides with my tarsnap backups. He (or Amazon) can respond to legal requests with my strongly encrypted data, and Neither Colin/tarsnap nor Amazon can provide them with my private keys.
You can design your systems this way. It appears you're allowed under US law. It seems there's companies jumping through hoops on behalf of the NSA and/or FBI to build systems that _dont_ provide that guarantee.
Note that Colin _could_ conspire with / be compelled by the NSA to attempt to convince me to "upgrade" my local tarsnap code with a backdoored version - and I'm OK with that, if the NSA is looking for me specifically, I fully expect them to find out _everything_ - that's their job and I expect them to be world-class at it. What I _dont_ accept, is that they have any "right" to record and archive permanently anything I ever do online "just in case". And I can and am taking steps to make that harder for them, and I'm noticing which companies are apparently working agains my wishes. I'm curious to know if Dropbox are noticing an drop in de-dup rates lately? My Dropbox storage is now all encfs encrypted - including the folders full of grabbed funny-cat-pics and Internet meme images. My versions are no longer the same as the other several million of them stored on Dropbox. Same for my SkyDrive/GDrive/Jotta accounts.