If anyone starts exploiting a giveaway thread then eventually the other users will spot it, and they will start reporting it as a result users will be nuked so no harm. But those which are genuine giveaways will be benefited.
Okay, but you run into the issue of introducing more to the workload of the moderation team for the sake of the few altcoin giveaways that are "okay".
Rather than a strict policy, now moderators have to take a look at things - whether there are helpful forum members or not, a report still requires a moderator's discretion when it comes to action - thus making it more difficult to manage the Altcoin sections. They're already bad enough: nearly every single thread in the discussion board is just absolute garbage and should be locked and trashed, and now reinserting giveaways only for the sake of a few?
I think it's something that would create more issues than resolutions.