While I agree with your free market principles, I do not agree with your application in this mixed economy context. Dropbox is not exactly a private company. The larger the company, the more it tends to be a branch of our government (via insidious corporatist mechanisms), so your standing up for their "private property" rights is dangerously misplaced.
An even more extreme example would be the telecoms, which greatly benefit from government privilege and where individual competition is prohibited. Saying "build your own telecom from scratch, don't complain" is just very deranged given the context.
Further, even if you were correct that it was a truly private company, your defense on the grounds of property rights is misplaced. A torrent is merely a technical way of copying efficiently, it is not a copyright-violating tool. Whether you believe it is often used for that purpose is completely irrelevant. What is relevant is that there are users with legitimate needs for this service, and Dropbox is arbitrarily declaring it off limits because some other users might use it in a way they don't approve of. There is no moral principle that says punish Peter for what Paul might be doing; on the contrary, behaving that is viciously immoral behavior.
So, Dropbox is indeed behaving immorally.