If they would open ALL the registers that are needed, I would agree. But when I walk in, and they're obviously busy, but only 2 lanes of the "manned" registers are open? Nah, I'm going to self-serve. That's a management issue IMHO. Although at Kroger for instance, sometimes the self-serve registers are slower, depending on 1) the intelligence of the people trying to scan their own groceries and 2) people that are buying alcohol or cigarettes (or anything else that requires age verification). Sometimes I'll go to a full serve register with one person in line rather than go to the self-serve if I see a bunch of people waiting for the ONE attendant to take care of the people that have to have assistance from the cashier.
That's WHY you go to the self serve and do the work for them... your impatience, and desire to get there and out quickly gets them your labor for free. They don't have to hire people, because people will work for free just to get out quickly. If people stood around creating a ruckus, or better yet, went elsewhere, they would be forced to open up more registers with live people working them. They have learned to use the apathy of the public to change the free market system to their advantage.