Airlines are legally allowed to overbook. It's an established practice.
Plus, I am not a lawyer, but I think IDB means just that -- boarding. Once you board the airline cannot change its mind, not for commercial reasons (although it might be able to weasel out of that by getting everyone off and re-boarding without some people). Again, take with a grain of salt; this is just what I heard.
Lots of things were legally allowed established practices until they were not; people realized they were unethical and immoral, hopefully people will begin to realize this practice is also unethical and work to abolish legal overselling
But overbooking is almost certainly right. Look, fact is that some pax won't show up for their reservation. Without overbooking, planes would fly emptier, wasting money and damaging the environment.
The airlines have fairly good predictions in most cases, and people have different preferences, so that almost always in the small number of cases where there is not enough space you can get people to get bumped voluntarily. It's really a win-win.