Many airlines require you to have an onward flight out of a country before they will let you fly into it. This allows you to fulfill the technical requirement without having to pay for a plane ticket you aren't planning to use.
A reservation that is not paid is not ticketed. A reservation that is not ticketed is not 100% guaranteed
But I agree that this is till a problem for airline inventory management, and an online travel agency should definitely not be opening this feature to the general public!
There are several usecases that require unpaid reservations, the most common one nowadays is corporate travel, when you need to integrate approval flows - the approval step will be between the reservation and the ticketing.
Like just about every industry, there is a world of complexity hidden underneath the surface :)
