What people don't want to do is to be constantly bombarded by bullshit communications from said brands. In many ways, openly declaring your brand preferences is signaling - the "liking" of brands is for their friends, not an enthusiastic wish to receive "special offers" from you. What they dislike even more are the highly intrusive ways brands are marketing themselves on Facebook.
It's great that you're running a promotion for a brand I like. No, I will not let you post to my wall as me. No, I will not spam 15 of my friends with your stupid offer before actually receiving said promo. No, I don't want to be forced to "Like" your stupid brand before I can even look at your page.
Brands, I want to receive pertinent information from you. If Starbucks is opening a new store in my area, I totally want to hear about it. What I don't want to hear are bullshit "news stories" concocted by your marketing department that is entirely content-free and screams like a cheap attempt to keep your name in my head.