[law]

I'll just chime in to post another point that was missed here. Arguably the most important (and controversial) principle in American jurisprudence is the freedom to enter into contracts. Dropbox has terms of service to which you manifest assent either directly (by clicking "I agree") or by your actions (that is, just by using the service). I'd venture to guess that in their terms of service is some provision that gives them the basis to remove accounts at any time, for any reason.

Courts have upheld these terms of service/use agreements in many cases; just googling ProCD v. Zeidenberg will give you more information, if you're interested.