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Too many users abused unlimited Dropbox plans, so they’re getting limits
(arstechnica.com)
This is a most excellent place for technology news and articles.
For your last sentence, Dropbox can't tell whether those are legitimate backups that the DMCA gives you the right to, or rips from a piracy site. Uploading data that's all 1's is just dumb and is designed to "test" the server, in the same way a teenager might test their stepdad.