iOS has to be tested on carrier networks prior to release just like Android has to be tested. No carrier can afford for every iPhone on their network to crash at the same time due to an update. Because the carrier would take the blame for the crashes, not Apple. Not to mention if you allow phones on your network that are not programmed properly can crash your network. Hence Verizon has such strict protocols. They are the #1 carrier for a reason.
Carriers DO test iOS betas, but they actually have NO control over whether or not it gets released. Apple releases the betas to partners, and if there are no glaring issues, they release the actual OS update. Period.
If you are Verizon and you DON'T test the beta, you might have issues and then iPhone users will be displeased with your network and go to a different one. So carriers certainly do NOT screw around with Apple releases.
If you are Samsung or HTC, the Carriers update the device so the carriers literally sit on them for 6 months after they are actually available.