It's actually a fair question. The online service still has hosting and bandwidth to pay for at a minimum. Paying them once does not mean that they will be able to pay for hosting and bandwidth for the next twenty years that I expect to use the product.
For scale, I've been using VIM for almost thirty years. Twenty years is not an exaggerated number.
One $10 - $50 sale per month won't pay for hosting and bandwidth. Not to mention developers' salaries, rent, insurance, electricity, equipment, and a few pence into my childrens' funds.
For scale, I've been using VIM for almost thirty years. Twenty years is not an exaggerated number.