Unlocking the bootloader and sim unlocking are two different things.
Unlocking the bootloader allows you to flash/install unsigned or 3rd party supplied parts/partitions (like recovery, modified or custom roms, etc), and that in turn allows you to easily root. Motorola gives out the Bootloader unlock codes for the Developer Edition X and some carrier editions (like T-Mobile and Sprint). For ATT and Verizon (non-developer), Moto does NOT give out the codes (likely due to carrier's wishes). No one has cracked the bootloader lock on the X yet, so to bootloader unlock the Verizon (non-Dev) and ATT X the Chinese Website/Middleman selling the bootloader unlock codes is the only way you'll be able to do this.
SIM unlocking allows you to use the SIM from other carriers in your phone.
Phones that are purely CDMA/EVDO only, like the HTC Incredible, Original Motorola Droid, etc, do not use a SIM and therefore there is no "SIM" unlocking for them. Those phones have an ESN burned into them. The carriers only accept ESN numbers of phones which were made for them (couldn't take a CDMA/EVDO phone from another carrier and use it on Verizon because VZW wouldn't allow the ESN). Verizon's phone's started using SIM slots when they added GSM Global Roaming to phones like the Blackberry 9930 and such. They were sim locked to Vodafone (Verizon's international roaming partner). They could be SIM unlocked for global roaming by calling Verizon's international department directly and requesting the SIM unlock code. Usually telling them something like "I am traveling over seas and I want to use a local pre-paid SIM when I get there), can I have the SIM unlock code for my phone?" would get you the code.
With the introduction of LTE, Verizon is using SIM cards for both LTE and GSM. Verizon's LTE phones are SIM unlocked due to the FCC rules around the 700Mhz C block auction they won years ago. The Moto X, Galaxy S4, Galaxy S5, and others are all SIM unlocked right out of the box. Other carriers like ATT and T-Mobile, ship their phones SIM locked, so to put a T-Mobile SIM in an ATT phone, you need to SIM unlock it first. The exception being something like the X where T-Mobile doesn't sell it directly, Moto does, and ships a SIM unlocked version with a T-Mobile sim.