system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are airlines allowed to overbook their flights?

Was on a plane this weekend that was overbooked by 16 seats. The wife and I made it on but a shitload of people had to be bumped and had no idea until they showed up to the gate. 

Edit:  Thanks for the responses all.  I guess I see the point why they do it, I just don't like it.
gpt
In law there is something called an efficient breach of contract.  Imagine you are a chair maker.  You have a contract with someone for a nice chair that they are to pick up on Wednesday.  On Tuesday someone runs into your shop, announces they are holding a vital meeting for a million dollar deal, and require a chair no matter the cost.  You sell them the chair for ten times its value and when your customer comes in on Wednesday you appologize that you do not have a chair for them and pay whatever damages they have - probably their damages are limited to the loss of use of the chair for a few days so a few dollars.  This is an efficient breach - the chair maker is better off, the customer who was supposed to have a chair has been fully compensated, and the guy who desperately needed a chair gets one.  

Same thing with over-booking a plane.  95% of the time it is win-win for everyone involved.  The airlines make more money on the planes which allows them to reduce prices generally (airlines are fairly low margin businesses), the customer who didn't show up is no worse off, and the guy who got the extra seat gets to where he is going.  In the 5% of the time when someone actually gets bumped they make it up to them (usually they do an auction going into the plane asking who will get off for 200 bucks or something - i.e. trying to find someone who would profit by the bump and volunteer to be paid for their delay).

Very occasionally they have to bump someone against their will, and very occasionally the chairmaker's wednesday customer desperately needed the chair, but overall society works out much better with efficient breaches than without them.