Who'd pay them for support if everything ran smoothly?
Regarding those examples:
1) For RHEL and Windows, paying is not optional 2) Mongo is so buggy most people need to pay for support in production 3) MySQL works well, but don't expect anything more than "forum answers" from Oracle support. Only 1 in 1000 companies paid for support according to the former president of MySQL AB.
Actually, companies are reluctant to pay for support for Open Source products, hence the business model tweaks of Redhat and the Open Core models of Nagios and other smaller Open Source vendors.