It is the carryons that make boarding stressful, where everyone worries that if they are not at the front of the line the bins will be full by the time they board. Also, they slow boarding as people stand in the aisles looking for an empty overhead bin or else trying to play Tetris to fit their luggage into an almost full bin.
From reading the article, it seems that may get the same tax benefits as the checked baggage fees.