Kinda. My Xperia Z didn't update to Kitkat when it was available. I bought it at JB-HIFI and therefore it came with Telstra junk on it. In /system/build.prop there's a setting called Customisation Number. So I used towel root and changed it to a stock number and the system updated to Kitkat and removed the non-uninstallable Telstra junk.

Telstra didn't update because of a battery issue. Which was true... the battery died quickly if you left the GPS on. I didn't use the GPS much so I left it off. (This issue has since been resolved, then telstra enabled the update)

Considering I didn't even have Telstra as a carrier. it was because JB-HIFI is a Telstra reseller that this happened.

It's because the Manufacturer agreed to only update when Telstra updated. It's all through the Sony update software.

I don't think carriers do anywhere. I'm confused.