They don't want you taking up seats that you don't use. I think they're expecting their planes to be pretty full during this promotion. If you're not gonna use the seat, they could sell it to a revenue passenger, and earn more. But they can't do that if you're just a no-show.

And also: the number-one complaint about this program is likely to be: "there were never any seats on the routes I wanted". People who book and no-show will unnecessarily multiply these complaints.

No they wouldn't. Jet Blue doesn't overbook their flights. If fact, they pay you $1000 if you ever get bumped. See their passenger's bill of rights:

http://www.jetblue.com/p/about/ourcompany/promise/Bill_Of_Ri...

I think it's reasonable for them to ask you to make a three day commitment or pay $100. Think of it as a guaranteed reservation.

Are you serious?

For the same reason that all you can eat buffets have small plates.

Some also charge for uneaten food, which is a more direct equivalent to the $100 charge.

I've never heard of an all you can eat buffet charging for uneaten food. Example?

Often happens at all-you-can-eat sushi places; personally I know of three or four in Montreal/Toronto where this is done.

They're the kind of places where you order small dishes off the menu, and you can just keep ordering them until you're done. No limit on the number you can order simultaneously, but if you call for the bill when there are uneaten plates they'll charge you some nominal per-plate fee for each one (this isn't for leaving one or two pieces behind - it's for when you order a plate of ten pieces of sashimi and eat two).

I've seen signage to that effect at buffets before. At least one of the Chinese buffets in Houston (of which there are about a billion) has such a sign, though I don't remember which one. I don't know that it's enforced...just a not-so-subtle way to say, "Don't waste food!"

These are how buffets are where I grew up. When I moved to the US, I was surprised that buffets here allowed me to leave food on the plate while I went to get another plate of food.

I just get more plates.