Ok.
Company X has capacity for 10 people to download 24/7. It gives those 10 people internet, but they don't use it 24/7. They use about 50% of the capacity. Now Company X is still paying the full price for their reserved capacity. They decide to give another 10 people access to the internet, expecting they will also use 50%. This way they can reduce the prices for everyone, as their capacity does not change, thus their running costs stay the same (besides the little extra admin). Now these new 10 people all use the internet 24/7. They make the service bad for everyone. How do you handle this problem? The easiest way is to warn these people and slow them down, or make them pay more for unthrottled.
Now people complain and say "but tell me exactly how much I can use?", "Give me exact rules to follow so I can plan and use accordingly" etc etc. To keep internet as affordable as possible for everyone, they have to keep changing the usage allowed per person because they still only have a fixed capacity, but customer usage keeps changing. The available capacity thus determines how much you can use, but it is ever changing, and that is why you will never get fixed number, unless you want to pay a lot more. If you wan't rules to guide you, just pay more for unthrottled etc. But you buy the same as the guy on the street, and then expect to be treated the same, but you royally abuse the shared capacity.
New users come and go. Tomorrow their capacity is reached, two days from now it is 10% below capacity. Now a psossible solution to this is to adapt every persons cap according to what is available. This month you get 50gb, next month you get 40gb, the month after you get 100gb, etc. but they can't sell you a product that changes like this for many obvious reasons.
Now how about we introduce a product, where your bill at the end of the month equals the amount of gb you used? The problem is the bigger part of the population can't comprehend a gb, and thus when they see their bill suddenly doubled next month... Its begging for a PR/CR nightmare. Secondly people can hit and run, by using the internet excessively, then complain at the end of the month and refuse to pay and demand to go to another isp. The easy solution to make life so much easier for everyone, and avoid costs on PR/CR that will carry over to customers anyway. is to introduce a simple easy to understand and worry free product. Its like the iphone: There is one, and its the same everywhere. They call it uncapped, but technically its a (changing cap or unknown cap) But you can't sell internet where you tell people you will get capped at some point, we just can't tell you when. So instead of capping/cutting you off randomly, they slow you down so you can never reach that capacity. Which is ideal because you still have access all the time
So yes the conduct may not be nice, but they can improve it, but the costs will carry over onto you.