Your analogy does not explain why the carrier do this, because it's the other way around: You get a flatrate for a device and they assume that you're not going to use it completely. If you use it with another device, though, you actually might use all of it.
So, by modifying your analogy, you get a flatrate for at max 1 cubic meter of water per month, but you may only use it for drinking and cooking. No one uses that amount of water for drinking and cooking of course, so the carrier calculates that you actually use much less. If all the customers do, that means $$$ for the stockholders.
Now, if you break that agreement and use it also for washing dishes, watering the lawn and showering, you will use much more. This means less $$$ for the company and that's why they want to forbid it.
What they are selling you, therefore is not a real flatrate, it's something like "1000 free* phone minutes! (*expires after 24h)". It's a crippled flatrate, where they try to make sure you don't actually use it or only in a minimal way.