This is good to know, although I suppose that clause in the FUP means that they could enforce whenever but I also guess that would probably cause a lot of backlash from customers so its probably not worth the hassle enforcing it.
I don't understand why networks, ISP's have things like this since fibre has essentially limitless bandwidth?
Mobile is not the same as a bit of glass, there are limitations to how much capacity they can have on the radio network, so for a start the basis of the assumption is wrong.
However, it's not "free" or "limitless" as such - to simplify things a little, sure you can send quite a bit of bandwidth over a fibre, but you need more and more expensive kit to go faster and faster over that bit of glass, and at some point that data has to go somewhere which isn't within your own network - via peering or transit. Peering generally is very low cost, but not necessarily entirely free, transit is usually not even vaguely free, and whilst capacity is considerably more reasonable than it once was, consumption is also rising, and someone has to pay for that.
You also have to consider that if I had say 100 fibres coming in from various places, all of which could pass say 100G, I now have to have kit that can handle 100G x 100 if I want no contention somewhere in my network, and the more scale you need, the less "free" or "limitless" it is.
The compromise is thus to have less connectivity than potential raw speed if 100% of capacity is used, which is essentially what everyone does, to bring cost down. That then means you potentially might need a way to protect yourself from scenarios where 20% of customers use 80% of your capacity, as that then does present you a problem.
All of the above is a very much stripped down version of reality, but ultimately it isn't quite as limitless as it sounds and the devil as always is in the detail.
In this particular case, EE has finite radio spectrum and capacity thus, and it will have finite backhaul capacity - I can assure you that they do not have every single cell site connected with insane amounts of capacity, for various reasons and again as a result they need a way to protect themselves from a small number of customers flooding the capacity of a cell site with limited capacity, even before you consider any other capacity issues elsewhere in the network, hence the FUP.