The average customer does not know what version of Android their phone is running. They wouldn't waste their time spending money for exclusivity on a software upgrade. From the beginning the only reason for a delayed HTC One launch seems to have been due to the fact that they still had plenty of Droid DNA's in stock and wanted to clear those out first. There was also speculation that Verizon initially had no plans to carry the HTC One due to poor HTC performance from the previous models Verizon carried and may have decided to carry it late due to positive reviews, but there was never any proof of this. Their getting 4.2 is just likely a benefit of being late to the game. HTC is notoriously slow with updates anyway. They launched the phone with 4.1 despite the fact that 4.2 had been out for more than half a year and if it gets 4.3 at all that'll be amazing - but it wouldn't happen until sometime mid next year knowing HTC if it happens and KLP will more than likely never come.
Realistically, 4.2 will launch for other US carriers around the same time Verizon releases the One. I wouldn't be surprised if HTC was just waiting for Verizon's to come out and launch it in the US at the same time.