Read your contract. You don't pay for a "5GB data plan". You pay for a "5GB data plan to be used on your mobile only". Violating your contract is not "free market".
What does "to be used on your mobile only" even mean? Does it mean you can't transfer any data downloaded on the phone to anywhere else? That's ridiculous. Or perhaps it means the connection must be via your phone, which is definitely the case if you are tethering. A data plan is an Internet connection. However you look at it, it's a gross violation of net neutrality principles to say what you can or cannot do with the data transferred through it.
"reduce congestion" is just a weasel way of saying that the providers sell beyond their capacity. I understand overbooking is a very common practice, but it is based on risk assessment -- and providers shouldn't be able to dodge all risk for their decision by disavowing their responsibility in single-sided "contracts".