You can’t abuse unlimited. That’s why it’s called “UNlimited.” I hate this two faced, corporate back sludge that always, and I mean always, puts it on the consumer as if they did something wrong. When in reality, it’s the company that is redlining or needs to boost those unsustainable goal of doubling revenue every quarter, ad infinitum.
The real narrative is Dropbox needs money so they are scrambling to cut every expense. No matter what spin they put on it.
You can DDOS using an "unlimited" VPS, and DDOS the same provider. Is that abuse? Of course it is. You can't expect a for profit to allow people to upload petabytes of junk all at once.
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.
Just violating the TOS, which means you are using a service or product outside its intended usage.
Downloading from a plan that has no cap, even if you download a lot, is simply making use of the service for its intended purpose. (Which obviously isn’t to DDOS someone.)
Why you’re defending DB here, a faceless corporation, is probably a better point of discussion.
You shouldn't try to benchmark some random server by uploading and downloading files that consist of the bytes FF repeatedly. Store all the crap you want, just don't ruin it for others.