I would argue that that your latter assertion is
not the case, at least not here in the US. When I got my first cellphone in the mid-00s, you bought your phone on an installment plan from the carrier as part of the service contract, the purchase installments were rolled up with your and service fees, and that's the way the world worked, regardless of which carrier you chose. Bringing your own device was not a thing.
It wasn't until later that I learned that cellphones could exist independent of their carrier networks and be unlocked therefrom; that one (in theory) could purchase a phone somewhere else, and then bring it to the the carrier of one's choice to be configured to interface with that network. And I only learned of that fact because I am nerd who likes to learn as much as he can about anything he becomes involved in.
Most people aren't nerds or have nerd-like tendencies. Most people simply want a magic rectangle so they can have Snapchat and Twitter in their pocket. The carriers do little to advertise the fact purchasing phones independently is even an option. So I'd say, no, most people don't know what they're getting into when they sign up.
And lest you say that said terms are right there in the contract, while true, be honest, when was the last time you carefully read and understood all 40-pages of the EULA when you have a more pressing problem to solve? I suspect even you commonly simply click the "I Agree" to get that damn boilerplate out of the way so you can Buy The Thing already.