You can't really sell something unlimited and then call who's taking benefit of that "abusers" :)
Colloquial versus contract. It's sold as unlimited, because for most people, it is. Similar to how salt and pepper at restaurants is practically unlimited, even if you can't demand they hand you all the salt in their kitchen.
In practice, marketing as unlimited to suss out the use distribution before capping it where it becomes uneconomic seems to be a valid strategy. (The fraction of users curtailed plays into perceptions of fairness.) With that framing, this story has no bad guy.
Neither do they advertise their lack of food poisoning. It's baked into the concept of a restaurant. Unlimited != infinite; it's a fuzzy boundary, and that is okay.
You're talking about unwritten AND unverbalized social contracts.
Entering a restaurant that has salt on the table is nothing like me entering into a written agreement for unlimited salt from a salt distributor.
What in Dropbox's terms of service do you think they're violating with this move?
Buying unlimited storage for common use fits into the paradigm of the not needing to be stated understanding on what unlimited means to a reasonable person. If you want to get more precise, the terms and conditions specify that Dropbox may, at its discretion, take various actions.
If I buy a meal from a restaurant and that restaurant provides complimentary salt on the table I do not expect unlimited salt.
Selling something as unlimited at a fixed price is clearly stupid unless there is a limit on the speed of the consumption (like the salt provider saying that you can have unlimited salt at a maximum rate of 1kg per hour or so). For something like dropbox it is even more stupid since every unit of product sold will increase their running costs.
all you can eat is not actually all you can eat
Really though, I find that buffets are all-you-can-eat with the asterisk that you can't stay more than a certain time (usually 90-120 minutes?) and that you can't take any home.
I don't remember who did it but I recall a skit that went the other way. Someone went to an all you can eat restaurant and to their dismay discovered it meant all you can eat. The restaurant would not let you leave until you could no longer physically eat any more.
I was never charged more because they decided I was full. Certain restaurants have a "no food waste" policy that will make you pay the stuff that you don't eat.