while I have worked in the telecom industry for the last decade in both a Linux/Unix and Cisco/Juniper troubleshooter capacity, I don't know enough about the mobile phone tech to give competent technical answer... but I will say that CDMA phones (Sprint, Verizon, Virgin, etc) have comparable hardware built into the phone that a GSM phone gets via a SIM card. The phone itself must be manufactured to exist on a specific CDMA network, that network needs to allow access to that uniquely IDed hardware (much like a computer MAC address) and then the phone's operating system must authenticate it's assigned account info.
While rooting my phone does allow me to use Wifi Hotspot and USB tether to gain internet access for other
devices via 3G (or 4G once my area upgrades) without paying an extra $30 a month for it, the reason I do it is to remove Sprint's bloatware crap (Nascar, NFL, Sprint TV, etc) and run a leaner, faster, more efficient version of Android.