Its a simple business tactic, "Lets have something that people want and no one else has". For example your favorite youtuber might be Laggin24x and you typically wait to watch any game play of a map until Laggin uploads it.
But sense yaw has a mapper in their pocket for exclusive content you will be forced to watch the content published by them to see the new map. Therefore simply boosting their view counts and subs.
As for the opinions on the quality of maps that they have exclusive access of, believe it or not these "parody" zombie maps are actually very large hits and receive a lot of views/likes for as boxy and simplistic as they seem. Where as here at ugx (as well as other CZ sites) I sense we hold our criteria for maps higher than the general public who do not create maps or have the opportunity to play any.