Cell phone companies get a government granted monopoly on a public resource (spectrum). The cell phone industry wouldn't exist as it is without government interference.
But there is nothing inherent in offering cell phone service as far as locking phones. That's a totally separate issue. I assure you that you could buy an unlocked phone (at full retail) and use it on a network of your choice with an appropriate SIM card.
You just don't get the telco to pay for 2/3rds of your phone when you do that.
This isn't really about cell phones, it's about copyright and how copyright laws apply to physical devices that you own. The fact that the devices in question happen to use public airwaves is incidental. IMHO, I should be able to hack my WiFi-only iPad and my XBox without risking criminal prosecution.
You just don't get the telco to pay for 2/3rds of your phone when you do that.