Software does actually require on-going maintenance and so constantly costs money to run, maintain and support. It's like buying a car and expecting it to be free every time you take it to the mechanic - it's obvious that is not economical. Things tend to be less obvious with software as it's more abstract, but many similar principles of engineering apply.
If a tool doesn't have a sustainable business model, it could end badly. If you're going to invest in learning and using a tool for a long time, you probably don't want the company to go under just as you're getting to grips with it. So a sustainable model is in the interest of everyone who uses the software, too.