It's also a licensing issue. phpBB Extensions and Styles need to be GPL v2, because they depend on phpBB code to operate. Selling your own GPL v2 code means that anyone buying it can then turn around and give it away or redistribute it for a fee.
I follow WordPress issues as well, and I know that plugin reseller sites exist that undercut the original authors. So yes, there are WordPress contributions for a fee available on sites outside wordpress.org, but it doesn't come without problems.
Be careful what you wish for?