They block this because, when you have 2 or more accounts logged on, you can make more gold than usual. Like multiple fishing, botting, "auto attack" on one character while the one you are controlling do a backup hit ( So it is almost impossible for the monster to hit, unless it has auto guard), Earning stars for rafting and killing monsters, while one steer and the other aim (Which is extremely hard and will need an experienced player to do that), enter a quest that must have 2 or more players ( Like for Mana Shield you need a partner, if your a boy you have to get a girl partner, if your a girl you have to get a girl partner), etc.
So this is why they don't let you multi-client
-Btw- I've heard that Sambox is a good way to multi-client POSSIBLY not in mabinogi, but other games like Allods,etc.