IRBs seem a good example of concern signaling leading to over-reaction and over-regulation. It might make sense to have extra regulations on certain kinds of interactions, such as giving people diseases on purpose or having them torture others. But it makes little sense to have extra regulation on researchers just because they are researchers. That mainly gets in the way of innovation, of which we already have too little.
I agree with Robin. Mistreatment of fellow humans should certainly be stopped. However, why should academic researchers have to go before a board any time they want to interact with humans, just because they are researchers?
For the majority of legal responses that our society makes, the approach we take is that people act first and then, if there is wrongdoing, the legal system follows up. For example, you don't get interviewed before you buy a gallon of gas. You get interviewed after a house burned down with your car parked outside of it. You don't go before a board before you grade a stack of papers. You go before a board after it is rumored that you told people what other people's grades were. Prior review is stifling.
People who defend IRBs probably assume that they will apply a large dose of common sense about what is dangerous and what is not. For example, surely the IRBs for an area like computer science will simply green light research all day long. In practice, it seems they look for work to do to justify their budgets. Witness the treatment of "exempt" research, where IRBs that have the manpower to do so tend to require review even of "exempt" research projects.
I can only speculate why such a useless and harmful institution persists, but a big part of my guess is that Robin's signalling explanation is correct. If you are the president of a university, could you ever take a stand against IRBs? Such a stand would have the appearance of signalling that you are soft on protection of humans. I wish that people would pay less attention to signals and more attention to results. Pay less attention to how many institutes, regulations, and vice presidencies have been created, and pay more attention to exactly how a university is treating the people it draws data from.