HGiles: because GOG is counting on some portion of the people who voted for the game buying it. Giveaways for votes completely subvert the point of the wishlist, which is supposed to measure how many people are interested in a game, not how many people like free games, and are actually harmful because it makes the wishlist less reliable for profit-calculating purposes.
I'm going to bitch about it some more. It's not just that a portion of the people who voted will buy it - I bet many people who are active on the forums enough to participate in the giveaways already got the game from other sources - and it's not just "I promise I won't complain if this game is released".
Rather, by comparing "I find the game appealing enough to look it up on the list and vote without any incentives" votes and sales figures for
released games, GOG can assess probable sales figures for a game they're considering, which they may or may not supply to the dev. And by artificially raising the vote count, people are making the game (to GOG) and GOG (to the dev) look as if they
underperform, sales-wise.
By engaging in statistically abnormal behaviour, such as voting for a game because of a thread,
people are promising extra sales they wouldn't actually supply, even if they plan to personally buy the game. As in:
GOG: "500 votes! That means 10k sales in the first two weeks!"
Dev: "Awesome! Where do I sign?"
Release! Yet Another Masterpiece!
GOG [thinking] "Only 2k? Looks like our users didn't like the game and/or the genre. We should feature fewer games of the sort."
Dev [thinking]: Meh, just another indie game store with an overinflated sense of self-importance. From now on I'll stick with steam.