I believe this goes hand in hand with certain types of customers. Purchasing software is often a long-term decision, but many people only need it once. In 15 days, they can complete one or two projects and then forget about it. With a 15-minute limitation, however, you are effectively encouraging them—assuming the quality of your product matches the price—to purchase the software.
So customers were satisfied anyway, but because of the bug their satisfaction did not last enough:-)