[ATREYU]

Consider this an OVERSIMPLIFIED example to the tentative thing:
A customer calls a business and asks the proprietor if he has an item. When the proprietor says he has one left, the customer asks him to hold that particular product, and that he will be in to buy it later on in the day. Now, minutes later a customer comes into the store looking to buy that one last item, and the proprietor says he is sold out. Now the first customer never shows and the proprietor is left with a product and no revenue for it. He had a chance to sell it.

Granted, in that situation, there are MANY things that could have been done alternatively, but the basic concept illustrates my point. A person posts tentative and when a person doesn't show, the host is left with less numbers, and is possibly out for recovering costs they may have needed to recupe for expenses.