So, if I rent out my bike to you for an hour a day for really cheap money and I do so a 50 more times to 50 others, so that my bike is oversubscribed and you and others don't get your hours, that's OK because it is just capacity planning on my side and widely accepted? Good to know.

Let me introduce you to Citibike?

Also, this is more like "I sell a service called take a bike to the grocery store" with a clause in the contract saying "only ride the bike to the grocery store." I do this because I am assuming that most users will ride the bike to the grocery store 1 mile away a few times a week, so they will remain available, even though there is an off chance that some customers will ride laps to the store 24/7. However, I also sell a separate, more expensive service called Bikes By the Hour.

My customers suddenly start using the grocery store plan to ride to a pub 15 miles away, so I kick them off of the grocery store plan and make them buy Bikes By the Hour.

As others pointed out, every business that sells capacity does this, including your ISP provider.

They could, of course, price your 10GB plan under the assumption that you would max out your connection 24 hours a day.

I fail to see how this would be advantageous to the vast majority of the customers.

Yes, correct. Essentially every single industry and tool which rents out capacity of any system or service does this. Your ISP does this. The airline does this. Cruise lines. Cloud computing environments. Restaurants. Rental cars. The list is endless.

I have some bad news for you about your home internet connection.