That's true in everything, if you want to be pedantic; every "free" service, like Dropbox's claim to have a "free" tier, is misleading because actually someone is paying for it (in that case, through a hidden "tax" on Dropbox's paying subscribers, some of whose funds go to pay for resources used by the free tier).
Dropbox is simply offering a product which is priced around where supply and demand meet. I'm sure if Dropbox believed that they could make more money by increasing or lowering the price, they would. By paying for Dropbox, you're not paying for the free tier, you're paying the price that you think Dropbox is worth. It just so happens that Dropbox can make a good profit at that price point, even with the cost of the free accounts.