Well, you do—until such a time as it may suit another party—perhaps. If the Megaupload case teaches us anything, it's that the inherent risk with cloud storage services isn't necessarily that they are not secure, but simply that once data leaves your network it is, by definition, beyond your control. And, while most services will clearly detail their commitment to keep your data safe, this may be no more than ink on a page when government or law enforcement comes calling—especially when you consider the data is likely to be stored offshore in jurisdictions with different laws from our own. The Patriot Act and Digital Millennium Copyright Act (DMCA) in the States, for example, has exemplified the vulnerability of data stored on US soil. Under the Patriot Act, there is no requirement that you be informed your data has been accessed, so you wouldn't even know. And DMCA guidelines can be found on many cloud storage sites for accelerating the submission process.
This may seem like a non-issue—after all, your business data isn't likely to raise the ire of law or government one would hope—but there's no guarantee when you don't even know where your data physically resides: many services use geolocated data centres, which makes it nigh impossible to determine what rights you have should a legal request—justified or not—be received by the service provider. The posterboy for cloud storage, Dropbox, for example, clearly states that employees can access your data if legally required to do so (though to be fair, this is true of most services, not just Dropbox).
Which raises another issue: how far do you trust the service itself? What's stopping abuse of access by administrators or other employees of the service? Would you even know? And this says nothing of a service being hacked through a vulnerability or socially engineered, or if it should be bought out by a third-party—then you'd be at the whim of the third-party as to the sanctity and sovereignty of your data.
And all of this is possible because, even though a service may clearly state that your data is encrypted both in transit and at rest, the fact remains the service provider usually has the keys. If they have the keys, the data can be decrypted, so you ultimately don't have full control over what happens to it.
It muddies further when you realise that many cloud storage services rely on outsourced networks such as Amazon's S3. While on the one hand this could work in favour of a given service if you know the pedigree of its partners (one provider even proclaims in its literature that its data centres are patrolled by armed guards!), it's yet another layer apart from having control over your data.
And this is the crux of the issue. IBM recently made the news when it announced that the use of Siri was banned for employees. Why? Because in processing voice requests, the audio is sent to Apple data centres—and while we like to think Apple wouldn't data-mine it, let’s face it, this is business (nothing personal, as the saying goes). For IBM, this is about limiting the inadvertent disclosure of sensitive information—a description equally apt for describing employee use of cloud storage services too. No surprise then that IBM has also banned the use of Dropbox and Apple's iCloud.
This is not to say cloud storage services can't play a role in your business, and most services offer specific enterprise class accounts with more features, but even with these you need to consider what data is to be stored with them, and most importantly, how secure it will be.