"the purchaser can then download the file from your storage via our website, so the file's location is never exposed." It looks like their site acts as a proxy to the dropbox link which only allows those who have paid to access the file
I suppose they'd say that this problem exists however you get the file to the consumer; their business is being the way you get it to them [legitimate users], not piracy-prevention.
But realistically, the 2 are intrinsically linked. I'd be interested in something like this that actually did something about piracy. I've built the systems myself, so I know it can be done, but it's annoying to maintain.