If your TOS states that Dropbox can't be used for files above a particular size, or can only be used by individuals and not corporations, or something like that, then fine.
But companies that discriminate based on the purpose of usage -- this seems fundamentally wrong. MasterCard blocking WikiLeaks payments, Dropbox blocking torrents, PayPal banning a VPN provider... Private companies have no business deciding what is "acceptable" in this manner. If it's illegal for them to provide services, then obviously they can't, but if it isn't, then they should continue to provide services.
Honestly, is this any different from a company ninety years ago denying service to non-whites because it "could be perceived as encouraging the races to intermarry?"
There's a reason we have anti-discrimination laws for races, gender, orientation and religion. Why don't we have them for "purpose of use of services"?