Imagine if the electricity board were allowed to supply you with "up to 240 volts" when in reality only supplying a degraded supply at 120V
Or how about, imagine if the electricity company were supplying you with an "up to 60/80/100A" service ? Well that's exactly what they do - your house (depending on the age, location, and local infrastructure) will have a 60A, 80A, or 100A fuse in the mains supply - ie the fuse that you can't get to, sealed inside the box on the end of the supplier's cable.
Can you use all 60/80/100A ? Well, sort of - as long as your neighbours don't also try it ! The supply industry works on an averaged demand of only 2kW (that's about 8A) per household - if every house decided to try and use it all at the same time then it would blow the fuses (possibly as small as 300A) at your local substation !
In our office building I happen to know that most of the offices have a 63A fuse protecting the submain from the meter room to the office. Down in the meter room, there are rows of meters (one per office), fed from a distribution box containing the suppliers fuses. There are 10 units fed from each phase in the box, and a 315A fuse upstream. So 10 units, if they all pulled 63A would be trying to pull 630A through a 315A fuse.
In general this isn't a problem - diversity of loads means that people don't all try and use all the supply capacity at the same time. There is massive variation in demand, and some correlation (eg most offices use power during 9-5 and little outside of that), but overall there is nothing like the capacity needed to be able to supply everyone with what they thing they are buying.
Elsewhere, there are some countries where your electricity supply has a much lower rated current - and you pay according to that limit which may be as low as 5A in some places, enforced by a circuit breaker provided by the lecky company. Mismanage your loads (eg forget that the washing machine is on when you try and boil the kettle) and the lights go out.
The internet is the same. There is massive diversity of demand (bandwidth usage), and no ISP could afford to buy enough bandwidth for the theoretical possibility of satisfying every user trying to use their "full amount" at the same time. The main difference is that (historically, it's changing these days) the electrical distribution system was designed by competent engineers, with a view to providing a reliable network. The internet is largely run under the control of beancounters with money the primary motive - ie not looking at "what bandwidth is needed to provide a decent service most of the time" but "how cheap can we go before the complaints get too bad".
Clearly different ISPs have a different view on what's acceptable.
meanwhile, in the UK lecky business, there's a lot of push now to cut costs - and that includes a significant shift from having spare distribution capacity to having disconnectable customers. ie if there's a major fault, instead of being able to route around it, they pick up the phone and tell some commercial customers to cut their demand - a facility for which the customer gets a discount on their bills, and a further discount when/if it's actually used.
And finally - "Smart Meters" are primarily about bringing this to the domestic market. Think 1970s style rolling blackouts when there's not enough lecky, but done on a house by house basis rather than block by block.