If I sell you something I don't actually have and cannot deliver, I risk serious trouble. It could be characterized as fraud, especially if I take your money then tell you I don't have it. If I tried to give you something you didn't actually want in exchange for what you wanted, it could be characterized as "bait and switch". Historically, these have not been good business practices and have landed many on the wrong side of justice.
In my many years of airline travel I've sometimes encountered "overbooked" flights. It's happened on multiple airlines and is somewhat standard practice in the industry. I didn't experience a lot of "overbooked" flights but enough to have a sense of how it works. In each case, I was traveling for business so I had a specific mission and schedule. Twice, I volunteered to take the next flight as it didn't mess up my mission but to the airlines benefit, I actually never took advantage of the very limited rewards they gave me. It was always flight credit and had to be used within a window of time. Since I rarely flew for personal trips, it was rather useless as the company had no way for me to apply these credits when booking flights.On other occasions there was simply no flexibility in my schedule to allow delays so I didn't volunteer.
Most of us don't fly more than once a year for personal reasons. The credits I got for the times volunteering to wait, required using the credits within 6 months. Even traveling for personal reasons, most people have a plan - a schedule - maybe a connecting cruise.
These days when we book a flight, we can even check in 24 hours in advance and select a seat if we wish on most airlines. We feel a sense of assurance that, short of emergency or weather issues, we will get to our destination reasonably close to our planned arrival. We have expectations - pay for something and we get the expected return.
In the past few decades, airlines have worked to streamline flights and assure that planes are full. They need to maximize their profits. When I flew in the 80's planes were rarely full of passengers. It was common to have lots of room and empty seats around you. Not so today. Now an empty seat beside you is a luxury. Airlines, working to ensure a full plane flies, overbook flights - sell more seats than they actually have - expecting that some passengers will change plans as often people do. In essence, they are selling something they don't actually have and gambling that, by flight time, they actually will. If they gamble wrong, they "pay" people to volunteer to take a later flight that isn't full. If enough people do, they are fine. And if their "payment" - the voucher for a future flight - is not used (like mine never were) they made a great gamble pay off. If, on the other hand, they have to physically pull someone off a flight and get sued for it, they just lost a big bet.
I'd venture that the overbooking of flights might be getting a second look in many airline boardrooms very soon. It's not a good idea to be selling things you don't have and betting that people will take the second option.