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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.
It depends on the ToS. DDoSing might be considered unreasonable use.
But if you're using VPS to stream 4K content 24/7, that would be heavy and reasonable use.
Similarly, if I take the unlimited Dropbox plan and resell it, that's probably against the ToS.
If I'm uploading 50TB of blu ray rips for backups, that's... Heavy use but entirely acceptable based on what they're advertising.
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.