Because that's what the rules say. If you don't like them then you don't have to buy the product.

Not to mention that the opposite applies. If you have a D ticket, and there is no business class, then why do you fly in first? Because that's what the rules say.

Youre getting a product at a discount. That discount comes with caveats. If you don't want to deal with those caveats then you can always just buy a full fare J or F ticket and essentially have little or no rules to deal with.