Simply because context matters. Again, Blizzard was painfully clear that leaving on it’s own wasn’t the cause of the suspensions. It was in conjunction with malicious intent. The person who said they left 90% of their keys also specifically mentioned that it was after a major group error where he felt the group would not be successful. He said he communicated with the group before leaving by saying something along the lines of “keys bricked, gg”. This is extremely common practice in keys above +12 and is accepted.
You have to remember that leaving on it’s own doesn’t flag you. Enough groups have to report you for leaving to even start the process followed by a review to determine frequency and intent.