Listen carefully KiltedTim, because you seem to be a little brainwashed by companies like Apple:
LTE is a network standard, just like GPRS, EDGE, UMTS, HSPA, etc. The purpose of such a standard, is that every device that is build according to that standard, is able to connect to every carrier supporting that specific standard. The European model of the iPhone 5 supports the LTE 1800 MHz standard that Belgacom/Proximus uses.
Now if a company (e.g. Apple) sells an unlocked phone that supports a certain standard (e.g. LTE 1800 MHz), but it doesn't work with a certain carrier (e.g. Proximus, which does support that same standard), onlybecause Apple enables/disables LTE support on a per-carrier basis using so-called "carrier bundles" (.ipcc files), and refuses to release a carrier update, just because Proximus doesn't have a contract with Apple, that would be illegal according to Belgian and EU laws.
I know that in your country, the USA, companies have all the power, but in the European Union there are laws to protect consumers and fair competition, so if you buy a product with certain specifications, it should work according to those specifications. Contracts with carriers don't have anything to do with that, nor do lists of so-called "supported carriers". I hope this cleared things up for you.