"If you buy software, you could easily duplicate and distribute copies (free or for a fee), but the agreement forbids that."

No copyright forbids that, you don't need a stinking EULA for that. The licence is more about "you can't do anything and there is no warranty".

"If you buy a mobile on a plan then you have agreed to take on the plan as part of the bargain."

Yes you have a contract, if you break it you have to pay a penalty. That has nothing to do with locked phones, that is all about blocking features they don't want to you to have (like blocking USB so you have to send photos to your self over the phone network at a fee) or to make it harder to switch companies after you finish the contract. My contract is paid off, I own my phone and if I want to take it with me to a different provider or buy a pay as you go sim when I'm travelling I consider that my right.