Why don't they just sell the unlocked international phones on their main website instead of having people go through authorized carriers?
Some do. Nexuses are sold this way. Motos are sold this way with customizations. Sony has sold its phones directly on its website before.
Given those examples (aka not very popular devices outside the Moto G) it is obvious why the Samsung's of the world don't do it: there is more money to be made through carriers.
First things first, by going through the carriers they get to ride piggyback on carrier advertising. Sure that means bloatware is part of the deal but when we look back we can easily see Verizon made Droid what it was. Heck some people still call all Android phones Droids.
Also in an unlocked phone world a Samsung actually has to compete on price with a Moto, Zenphone or a OnePlusOne. In a carrier store where every decent phone is $199 they only have to compete with Apple (which EVERYONE does anyway). A carrier phone means $HUNDREDS of that S6 price is margin that goes to Samsung, in the unlocked market a OnePlusOne is barely making any money on each device sold.
It is night and day the difference in profit, which then gives us better products. In fact without the carrier subsidies I fully believe there isn't a phone on the market as nice as the S6. It would be like Windows laptops where each one has a few major flaws caused by a corner cut here or there.