Not to mention, it's potentially confusing for a consumer who happens to come across the "free" version of your game and may not be aware it's pirated. I still read comments from people who are shocked that RPG Maker now costs money when it used to be free, not aware that the 2K and 2K3 they're referencing is in fact pirated and was never free at all. In this day and age of freeware games (especially when it's an RM game), it's reasonable to assume a certain level of confusion will occur amongst consumers who thought a game was free when it was not.
And even if a consumer was aware and planned to legitimately buy the game, if the same game is dangled in front of them as being free and they're fairly confident it's not infected with a virus, or that they're anti-virus software is top notch if it is, you can't tell me a good fraction of those consumers won't be tempted and get the pirated version instead, even if under the false pretense that they'll still buy the game at a later time if they like what they see.
So yeah, I can sympathize with the commercial developers. Pirating the game may not amount to a big deal in the grand scheme of things, especially since the pirate wouldn't have purchased the game anyways. But posting the pirated game and then luring a potential crowd of would-be buyers is a big deal, since that single lost sale begins to add up with each person who chooses to download the pirated version instead of buying the legal one. It's even more of a concern for indie developers without the know-how and resources that a bigger gaming company will have to fight this.
Edit:
To add one more thought on this, I do realize there are some people who will download a pirated game, like what they see, and then go off to buy it. So to some degree, it is a valid point. However, it's really no different than reading reviews, watching some brief play-throughs, or even playing the demo and then deciding whether or not to make a purchase. So even without a pirated version, those people who would go out and buy a copy of the game would probably already do so with the other legal avenues they have.
However....
You can't be naive to think that every consumer who downloads a pirated game will then go out and buy a copy, whether or not they intended to in the first place. So even if you had 10 potential customers who each downloaded a pirated game, and only 5 of them decided to buy it afterwards, that's still 5 missed sales opportunity on the remaining 5 who didn't go out and buy the game. We can't know if those remaining 5 would have bought the game had there not been a pirated version, but it's reasonable to assume that some of them probably would have. It's pretty much guaranteed that pirated versions will almost always cost the developer sales more so than if there weren't a pirated version, even if a fraction of those people decide to buy it afterwards.