I disagree with your characterization of this practice as "stealing." Your argument takes for granted that it's legally impossible to sell something with the proviso that it must be used within a certain amount of time.
But you're simply wrong about that, legally speaking. The contract you signed does not say that what you're buying is some permanently enduring allotment of 10 GB of data per month. It says that what you're buying is the right to use up to 10 GB of data in a single month. There's nothing illegal or even, IMO, immoral about a sale with the proviso that the good purchased must be used within a certain amount of time. If you don't like it, don't buy that good/service.
Well we must agree to differ.
I am not saying that the practice is illegal, like actually stealing. We have after all signed a contract which details that they will take bandwidth that we don't use. And of course it is a contract of adhesion, so we can't go negotiating the agreement, we have to take it or leave it.
And while it would be nice if there were dozens of phone providers to choose from and fierce competition kept prices down, that's far from the reality in the US. Essentially there are two massive dominating companies, and a couple of smaller ones. The former offer better coverage, but crappy expensive plans. The latter have better plans, but less coverage.
Whether you choose to believe me or not (feel free to look it up), we here in America do not get very good value from our mobile phone companies. In many other developed nations they have far better pricing.
As for your opinion on the morality of this, I think that if we pay for 10Gb of bandwidth, there is no reasonable reason as to why the phone company has to confiscate it if we fail to use it within a month. The only motivation for this and the use of overage charges is to make money. It's like when the banks arbitrarily fine you if you overdraw your account. Sure, they want people to not spend more than they have, but the $38 billion dollars a year they make in extra revenue probably feels pretty good too.
In 2013 the major players including Verizon started pushing data sharing plans, but it turns out they secretly changed the math to make data even more expensive, despite the reality that the cost of bandwidth to them is and has been falling.
And just look at the lengths they went to to force people off the old unlimited data plans. Turning off WiFi hotspots, disabling various other features, and generally doing anything they could to get us to buy more expensive plans than ever before.