Accounts being Blocked.

But do you personally find it a morally incorrect thing to do, is what I'm asking. Every game's ToS states not to do that, even ones that make cash shop items completely tradeable or have in game means of doing so.

It's not a question of morals, it's a question of rules. While I agree forcing players to buy only from them is kinda' dickish of nexon, it's still their right as a business to not want people to undercut their income.
Let's put it this way.

Player A wants to spend 12k on a cube package
Player A would have to spend $20 on NX to get the cube package via prepaid cards
Player B has 20k NX
Player B offers to sell the cube package to Player A
Player A agrees to the trade, and gets gifted the cube package

Player B had the NX regardless
Player A now no longer needs to buy the prepaid cards
Nexon loses twenty dollars in potential profits