This is actually incorrect. Verizon's LTE phones come factory unlocked. AT&T phones come locked. Verizon LTE phones have GSM/CDMA chipset so it can be used on any network out of the box, including AT&T and T-Mobile. AT&T can't be used on Verizon or Sprint because it does not have CDMA.
So in reality Verizon is more open than AT&T in that their phones are unlocked and they are GSM ready.
try to put a Nexus 5 on Verizon's network. case closed.