People who use root to cheat the system and get free wifi hotspot and other services are the reason they don't really want us to get rooted. I'm switching carriers because Sprint locks good people out of their phones. T-Mobile and the others are better about letting customers root their phones.
Your premise makes no sense. There are no carriers who "let" you root your phone. It's all on the manufacturer. What other carriers do anything less to hinder you from rooting?
I have an HTC phone. My wife has a G5. We are both on the same Sprint plan. But HTC allows me to unlock my bootloader which makes rooting a trivial process. LG has done a great job of locking the the G5 down so very few models have been exploited. This is great from a security standpoint. But Sprint has nothing to do with it. My previous phone was a G3, and I had it rooted and BL unlocked 6 ways from Sunday. Sprint didn't make that process any easier or harder.
Switch carriers if you want, but don't think it's because TMo will give you the green light to root your phone. They don't control it.