Re: A side-effect
"And not many other providers use band 13."
The problems come down to antennas and receivers. It's easy enough to make radios operate at any given frequency in this age of software-defined oscillators, in the same way you can tie a string to a manhole cover and fly it as a kite given a strong enough wind.
Having enough sensitivity to be USABLE across those frequency bands requires good matching of the aerial and decent tuned circuits out front. Wideband antennas/frontends necessarily lose sensitivity due to the drop in Q and there's only so much you can regain with software-defined tunable inductors.
Covering ALL the bands makes for expensive RF sections. Covering the most common sets is much cheaper. It'll change eventually - just as 900/1800MHz phones became the norm and then quadband phones became the norm later instead of costing 3 times as much as the dualband ones. But it'll only change when there's sufficient market demand to justify selling them - and if Verizon have Band13 sewn up worldwide, then few makers have incentive to bundle support for that band into non-Verizon-specific phones.
It works the other way too - if a telco has a world-unique frequency allocation, their telco-unique phones will be more expensive than standard ones for the same feature-set, which is a barrier to purchase if a competing telco is using "standard" bands. (Competition is a good thing, etc)