A theory...
...the radios in Spectrum, D4 and perhaps some of Verizon's other new LTE phones have the ability to use the GSM frequencies used internationally & here in the US by ATT. These phones will work on those networks now if one uses a GSM sim card in the LTE sim slot (still might require a hack to get data, from what I've read). This functionality is unannounced because Verizon's software and current batch of LTE sim cards aren't capable of switching from LTE to GSM. So Verizon isn't able to switch the phones to global mode with a quick reprogram... in other words, they're not able to charge users for global roaming, and don't want the masses to figure out that they could pop in a prepaid foreign sim card. If someone figures out how to program the phones to get full functionality with a foreign sim card, Verizon will lose alot of $$ over the life of these phones. Verizon is working on software and a dual LTE/GSM sim card that will make these phones work within their current pricing structure.
Complete speculation, but it's the most reasonable explanation I can think of. Is it complete BS?