I can easily see how the Toronto Library Board didn't want to wander into the minefield of determining which espoused opinions of a group make them worthy of being allowed to rent rooms in a public library. 

Unless the event is explicitly racist or in some other way completely objectionable, far better to let it go ahead and deal with objectionable conduct/speech later. 

The group that staged this is undoubtedly pleased at all the free publicity it has received as undoubtedly the vast majority of us would otherwise never heard of the event.