I also have a Steam account, so I taught it would be nice to activate this game as well on Steam. Where can I find the key?

Right, good to know, thanks!

GOG wants to lure people from Steam, so it redeems "backup" copies, but Steam has no need to lure people from GOG, so it demands payment.

Makes perfect sense, though perhaps not with goals we would like.

I understand the logic behind that. What I don't is that they've effectively made it so that purchasing the game on Steam gets you a key both here and there, whereas purchasing the game here only gets you the GOG version. That seems to incentivize buying elsewhere, as if you have the choice between the two stores and identical prices, it only makes sense to go with the one that nets you both versions.

Now, if they had made redeemed copies "lesser" versions somehow, perhaps by creating a separate version in the same way they have an Australian censored version and then removing the free extras (which I think are also free on Steam; ideally, they wouldn't have been) or adding new extras that are only available on GOG, that would accomplish the same thing without making purchasing on another store the better option.