"What's the point of being in business if you give your product away?"
This isn't talking about products that they sell, it is about internally developed and used software. This software could be expanded for use in other areas and become useful to other entities.
Most of this internally developed software gets to the point where the developer/company wants it to be and development stops whilst the software just does its job. This could be passed out to the development community and expanded into areas that weren't originally envisaged by its developers. This is free software improvement and upkeep for the original company that had the forsight to pass development of the software to the open source community.
"99.98% of the developers of open source software couldn't market or sell themselves out of bed, much less a successful marketing campaign."
Well, 99.98% of goat herders couldn't either. Why do developers need to be good at marketing? They are developers, not marketers. Leave the marketing for those that do it.