The Customer Support guy writes it clearly in the official forum:
We asked the GM for their latest POV on that question, he didn’t say queue-syncing is illegal in any form.
Again: You can be mad about it, but stop claiming it is against ToS or CoC. Blizzard has stated it is not. “Oh it is not entirely clear from that answer!” Well, then show me a single Blizzard statement where it is entirely clear that queue-syncing is, in fact, cheating, I will wait.
Yeah, it is. Like many things with this game. But it is not illegal (in contrast to the boosting groups in RBG and Arena…)
And, at the end of the day, it counts what Blizzard DOES. Namely not a single ban for queue-syncing in the last 20 years. Saneko, as Forum MVP, gave you already a good answer to the question: