"unlocked phones" mean that you are not restricted to using one service provider and you can just change your SIM cards to subscribe to a new service.
But this still does not mean that you can change the operating system of your phone hardware.
This is the bootloader. Unlocking the bootloader gives you to freedom to flash community developed Android variants.
HTC's commitment is to unlocking the bootloader - not the service provider SIM unlock (though you might be able to do so - however they are NOT the same thing)
"unlocked phones" mean that you are not restricted to using one service provider and you can just change your SIM cards to subscribe to a new service.
But this still does not mean that you can change the operating system of your phone hardware.
This is the bootloader. Unlocking the bootloader gives you to freedom to flash community developed Android variants.
HTC's commitment is to unlocking the bootloader - not the service provider SIM unlock (though you might be able to do so - however they are NOT the same thing)