As an end user, I understand there are moderators in a public forum to prevent spam and other stuff that is irrelevant to the plugin/theme in question.

But what’s the problem of providing support for a commercial addon/upgrade of an otherwise free plugin on the WP support forums? Why is it not allowed?

The support is provided by the developer and other helpful people who use the plugin. Nobody else is required to help.

I think it’s very inconvenient that there are multiple places to get support for anything around WordPress. It means having different sites, different systems with a different login for each plugin/theme you use. It would be so much easier if all support was handled on a single system that works the same for every plugin and theme, where I have all my support cases in a single system.

I do understand though that the larger vendors will want to have their own support system, because the WP support forum is a just a simple forum, not a full fledged support system. However, most plugins in the repository are from small developers who created a plugin to solve a problem and decided to share it for free. If they come up with a paid addon, then why would they have to get their own system if the WP support forum works for them?