This wouldn't be an issue if Reddit always attached relevant posts, including negative ones even if those were the minority, to actually help people make a more informed judgement about an ad based on community sentiment, but I think we all know that won't be the way this goes.

Posts will inevitably only be linked if they are positive, or at the very least neutral about the product being advertised, because that's what would allow Reddit to sell advertisers on their higher ROI. The bandwagon effect is a real psychological effect, and Reddit knows it.