It only takes one paying customer to take the published FOSS code from the commercial software and re-distribute it for everyone to benefit from the commercial modifications made to it. That's the point, a commercial use of the software can not make the source proprietary.

This is what Redhat recently found out when they tried to hide their RHEL source behind a paywall. Attempting to tie the hands of their customers with an additional license agreement forbidding distribution of the source is a violation of the GPL.