I too have never understood why DPS get a free pass compared to Healers and Tanks. Then people wonder why their dps queues are so long compared to them...

As a healer if people die 90% of the time I will get flamed,
As a tank if i lose agro consistently I will again get flamed and told i suck ect
As a DPS? 90% of the time people dont notice and if they do need to be careful what they say because parsers.

Also doesn't take into consideration how easier it is for a bad player to skate by as a DPS compared to a tank or healer.