I see what you mean, but this isn't exactly about modifying vs. using, and the hammer analogy wasn't very fitting in the first place, which is why I thought up the hammer factory (yeah, nah, not a 100% fitting analogy, either, I know). The thing is that Wordpress isn't just a hammer that was manufactured and now there it is. What we're talking about here includes continued work being put into it in the background for eliminating bugs, fixing upcoming security issues, adding improvements, and churning out new, improved hammers every few weeks. Using Wordpress as a tool to provide services for profit means continuously profiting from that work, without reimbursing the producers for any of it.
Then again, it's absolutely correct to state that the Wordpress makers have said "here, we've made this software, you're free to use it".
And that's the dilemma, because if someone suddenly makes millions off of that ongoing work of others, the idea that those who actually do that work would deserve at least a small piece of that big cake is not completely misguided, either. It's just that I can't see them succeed in enforcing that, I don't think the free and/or open source licences used will facilitate that.