Someone might follow your brand on Twitter, but if he doesn't check his feed for a while, he doesn't see your marketing-speak unless he goes looking for it. Unlike Twitter, Facebook actually tries to be intelligent and show you the stories you care about instead of defaulting to chronological order only. Since, shocker of shockers, people generally prefer to engage with their fellow human beings as compared to these quirky entities we call "brands," the stories you care about are more likely to involve an old friend getting married or your buddy's weekend pics than whatever inspirational message your social medial engagement intern shat out this hour.
Basically, Facebook is treating brands a lot like people: not all their friends see all their content in their newsfeeds, but some do, and higher engagement and closer connections make it more likely that someone will see your stuff. Brands, naturally, don't want to play by the same rules as us mere mortals and want all of their "content" displayed prominently in their subscribers' newsfeeds. In other words, it's Citizens United v. FEC all over again, except no one is actually limiting corporate speech here, just asking brands to pay rather small amounts if they want more reach and distribution than the rules allow.
To sum it all up for Mark Cuban: the mere fact that someone has liked the Mavs on Facebook does not mean that she wants to hear whatever your marketing interns come up with every couple of hours. The obnoxious guy who talks about himself all the time doesn't automatically get a free megaphone to beam his messages into the minds of everybody he's engaged with. I realize that you, Mr. Cuban, have made a career out of being precisely that guy, and that people do walk around hanging on your every word, but us normal people and normal brands have to pay for our promotion, and it's unreasonable to think otherwise.