Pretend I'm a user with a paid Dropbox account. I share a folder to 10 people and ask them to upload files to this folder for me so I can work on a project.
None of the 10 people has a paid Dropbox account.
I created the folder.
I own the folder.
I want the files they give me.
It makes sense that the files they provide me count against my storage limit.
It does not make sense that all 10 people now have all 10 users' files collectively measured and each of those 10 people now has all 9 other people's uploads count against each of their storage limits.
That's what Dropbox is currently doing to me. And since Free accounts aren't allowed to contact Dropbox (support is for paid accounts only) you have set up a system that harasses potential customers and leaves them with no option but to either throw money at you or stop using Dropbox, tell their clients not to ask them to use Dropbox, and block the series of "no reply" e-mails coming in from Dropbox telling them that other people have pushed them over their storage limit. Without that, every client I upload to whose shared folder is larger than my storage limit will trigger Dropbox punishing me for the client's storage, which is what's happening now.