I believe they added that somewhat recently after their privacy issue fiasco.
There is also this snippet that was added, which assumes that your key isn't really private as opposed to services like SpiderOak which have no knowledge of your key:
> We may disclose to parties outside Dropbox files stored in your Dropbox and information about you that we collect when we have a good-faith belief that disclosure is reasonably necessary to (a) comply with a law, regulation or compulsory legal request; (b) protect the safety of any person from death or serious bodily injury; (c) prevent fraud or abuse of Dropbox or its users; or (d) protect Dropbox's property rights.
There is also this snippet that was added, which assumes that your key isn't really private as opposed to services like SpiderOak which have no knowledge of your key:
> We may disclose to parties outside Dropbox files stored in your Dropbox and information about you that we collect when we have a good-faith belief that disclosure is reasonably necessary to (a) comply with a law, regulation or compulsory legal request; (b) protect the safety of any person from death or serious bodily injury; (c) prevent fraud or abuse of Dropbox or its users; or (d) protect Dropbox's property rights.