They aren't selling a product (open-source software). They are selling a service - the effort to customise the distro and package it with free, useful softwares. Hopefully, they also donate some time and money back to those free, open-source softwares. Note also that GPL has never been hostile towards commercial software. In fact, with MySQL (before it was owned by Oracle), the FSF even endorsed MySQL's dual-license open-source business model.