cheats; i.e. methods not expressly authorized by Blizzard, influencing and/or facilitating the gameplay, including exploits of any in-game bugs, and thereby granting you and/or any other user an advantage over other players not using such methods;
engaging in any activity that grants an unfair advantage is considered cheating.
This is related to the purpose of a matchmaking system, and the spirit of competing vs. opponents for “greater gains” from the match.
In a matchmaking system there’s something called a theoretical chance to win. This is far from 50/50 in the random matchmaking, and closer to equal in a rated system like arenas (or chess, which these systems are based on).
So the random matchmaking needs to be random because that is “fair” as in equal chance to get a favored chance to win in the matchmaking. Such as by setup or skill of players or being subjected to the same social circumstances, there are many things which affects it beneath the surface which is decided in the random matchmaking itself.
When only one side can bypass the nature of random matchmaking and go as a premade -
-it naturally makes the theoretical chance to win manipulated in your favor. In other words, it provides an advantage which your opponents can’t perform, which by its very definition makes it unfair, which is prohibited in Blizzard’s CoC.
(Keep in mind that unfair refers to equal opportunity, not unfair as in " 
multiple opponents ganked me while I was alone, in wpvp". Wpvp is designed to be chaotic in order to emulate a real “world”, while instanced PvP is not.)
Which is why they should just enable queuing as full premades with a Join as Raid option for AV as well.
There are social benefits to allow queuing as a full premade as well. It’d also isolate the ones only queuing to AFK inside AV.