Ok not exactly the same as a restaurant but let's say he's booked multiple, and I'm told by my dealer my car won't be in this batch because there's only 100 and they're all taken, so it wont be for another 6 months. So I've not made plans to receive the car and maybe i had to get another one until its ready or I haven't got the cash in place in time.
Next minute they call and say someone cancelled you can have it next week, I've not prepared to buy the car yet and therefore I loose the purchase, all because your friend pre booked more than what he intended to buy.
In summary people doing that sort of thing is annoying because it completely messes up people knowing when they can receive it.
Maybe a better analogy would be running down to the pool and placing loads of towels out around the pool. You don't need them all but you'll reserve them. I then walk down and go oh no they're all taken, ill have to make plans to take my family somewhere else then... then your mate takes half the towels off. They will be taken because its a popular place, but not by someone who already made plans to sit around the pool all day because selfish people took more than they needed.