

Two additions:
- some software is also sold under a separate license in addition to GPL, so you can choose if you want free open source or paid closed source (e.g. Qt has this model)
- being in breach of license terms doesn’t mean you’re forced to do anything automatically: it means you can get sued and a judge might order you to do something (probably pay money) to heal the breach, or you can settle out of court (probably by paying money).



You gotta admit that’s a huge market.