The reasoning behind the rule is that it's better to either a) be civil or b) contact support, or c) let an invalid giveaway run to completion and correct it later, than have a hostile forum/comments section with accusations flying about fake giveaways.

90% of the time, based on what I've see, someone confuses "Create a new giveaway" for "Wishlist this game"...somehow. You're certainly allowed to suggest to them they may have made a mistake, seeing as the game is on their Steam wishlist (as in this case), but accusing someone of creating a fake giveaway is akin to calling someone a cheat in a poker game - it's a big breach of etiquette.

For what it's worth, five months ago someone was giving away XCOM 2. I noticed he had it on his wishlist and pointed that out to him. He replied that he had played it to completion, just not under his Steam account, and wanted people to enjoy it as he did. I just checked the giveaway and the winner marked it as received. That gifter's given away Fallout 4, Civ VI, Tyranny and a few other games since. I made my comment without making an accusation, the gifter wasn't offended, and everybody wins.

Although I did not witness it firsthand, I also recall hearing that there was someone from Southest Asia whose first giveaway as a copy of Call of Duty: Ghosts (which was a much hotter title at the time). He got a lot of accusations of "Fake!" and, as you might imagine, didn't feel very welcome. Although I can't confirm firsthand, I recall hearing that he honored his giveaway(s?) but quit the site, feeling harassed and that the community was hostile.

SG may not seem like it gets many new, triple-A games like this, and fewer still from newbies, but it's exactly because they're rare that the SG crew wants to give everyone a chance. If that means temporary bans on people making accusations of fake giveaways and letting some mistakes get through (again, helpfully pointing out possible confusion is okay, accusations aren't), the moderators err on the side of not harpooning possible whales (big donors) and keeping things civil. The domain and code aside, what makes this site is the community, particularly people who get involved as gifters and constructive participants in the discussions section. Sometimes we lose those people to personal reasons that can't be helped, but I've seen a few lost to a sense of hostility, ingratitude, and harassment, and that's what the moderators want to head off.

tl;dr - The mods feel enforcing civility via temporary bans does more to preserve SG's sense of community / gifter participation than letting accusations fly does to filter out the occasional bad giveaway.