It's standard SG policy to inform of ongoing promotions; the GA creators who obtain their keys from such and don't make mention of it are already managing to avoid a favorable impression. If they start blacklisting for other people doing the courtesies on their behalf, then there's really no loss to being blacklisted by them anyway. Meanwhile, a GA creator posting a legitimate giveaway shouldn't have any issue with information that may be of interest to entrants, unless making such comment violates a listed request within the GA creator's giveaway description, one which specifically asks for users not to leave comments.

Meanwhile, everyone else'll generally appreciate the gesture, so the risk of an outcome that actually harms the commenter is pretty low- unless the commenter violates SG policy by explicitly casting doubt in if a giveaway is legitimate or not, at which point they're at risk of suspension and negative response.