It's because the 'for' crowd is enthusiastic, so advertisers want to channel a little of their enthusiasm to promote a product. The 'against' crowd is largely apathetic and doesn't associate with 'anti' particularly strongly compared to the 'pro' crowd.

On the flip side, anything perceived as anti-gay in business blows up with negative press and talks of boycotts. With Apple's demographic, this will definitely be considered a plus.