Because withdrawing from a queue over and over and over again is SO hard for them to detect, and automatically flag for investigation so they can discipline the trolls. They already have tracking stuff for the much more complicated problem of RMT. That sort of behavior should be ludicrously easy to detect.
And that's if it's even really that pervasive to begin with. You're telling me, that there are so many trolls that will group up because they're bored and click queue/withdraw over and over and it's so widespread that it will even affect your average player? I seriously doubt that. People queue/withdraw spam primarly for a reason, and that was for in progress dungeons. I seriouly doubt half of the player base has even had a single queue stopped because trolls group up to harass people they couldn't even see. And how many of them are coordinating their trollish behavior out of game because if they discuss their trolling in chat that's easy to show intent to harass and ban.
TLDR: what you described doesn't happen on a wide enough scale to even warrant action. And even if it was, it should be very, very easy to automatically track and ban.
EDIT: why did they use this solution? Because while they were solving the solo in progress withdrawl problem (badly I might add), they tacked this on here too without much thought whatsoever.
