These threads are fascinating.
I think we need to distinguish between the moral and the legal arguments, otherwise everyone gets very confused.
As mentioned, it cannot be immoral to obtain ideas from a work subject to copyright without compensating the copyright owner (hereafter: author). This actually leaves me wondering what it is that is morally wrong about copyright infringement.
It is also worth noting that - at least as enforced - in most jurisdictions in the world, copyright infringment is an issue of private law. The wronged party can sue for the damages caused by the infringement. The US has taken steps to make it quasi-penal: the prevalence of punitive damages in US tort law makes very little sense to me - but it's unusual in this. In most jurisdictions, copyright infringement is as 'wrong' as breaking a contract. And plenty of legal economists will tell you that often the right thing to do is to is to break a contract, and pay the damages. The law can tell us very little about the morality of copyright (which isn't surprising, given how recent an idea copyright is).
Copyright also only protects the expression, and the right to develop works from that expression. This is why the OSRIC-like products are (probably) non-infringing, and why recipes are not copyrightable. Coca Cola have to use trade secret laws to protect their ingredients and forumlae. Copyright would not help them at all.
Conflating copyright infringement and theft does many undesirable things. It suggests that the law has assigned a moral status to infringement. It suggests that copyright infringement is obviously criminal. And it devalues the 'theft' concept, which is depriving another of a thing which they are entitled to possess and enjoy. Infringment does not affect the copyright at all.
Though NB: copying a book out by hand is just as much copyright infringement as using a photocopier or scanner, unless fair use/fair dealing or a license allows it.
In short (and this goes for games, films, music, etc alike) I'm firmly convinced that private law is entirely suitable to deal with copyright infringment. Copyright owners whose rights are infrigned ought to be entitled to recover their actual, provable damages, i.e. the actual, provable amount of revenue they lost due to unauthorised copying. And no more.