Service and support. Suse has a paid enterprise version last I checked. Same as RHEL and Canonical. Nowhere in any FLOSS licence is there a clause that mandate free distribution: if you take time to select packages, create a cohesive distribution, test it, and test and apply fixes and security fixes might as well get paid for it, especially if you cater for enterprise which usually has more stringent requirements regarding uptime and security. They also sell service packages with very stringent service levels. that is as far as I remember, I think some develop custom solutions for a fee and then just push the code upstream, but I’m not too sure.

On top of that few of the open source licences (just the GPL and a handful of others) mandate source code distribution and IIRC just on request. GPL is virale as in software built using GPL code must be released with a GPL compatible licence, but even then you only have to disclose the source code when asked for it.

All of the above is at the best of my recollection, so take it with a grain of salt.