That's because you'll star in them. In the "My Player" career mode, if you become a superstar you'll be pitched to appear in official league advertisements, including the league's well known "Where Amazing Happens" campaign, with you as the amazing happening.
2K Sports' Ronnie Singh tweeted out that screengrab about 90 minutes ago, showing his player having ascended from 20th overall draft selection to household name. Singh said he saw the ad — a video, not just a still — during the third year of his player's career, so it's not like NBA 2K12 throws you this honour as soon as you have your first 20-and-10 night.
Text accompanying the image on the game's Facebook page suggested this was not the only advertisement you'd appear in, though it's unclear if all will be on behalf of the league or if some will involve actual in-game sponsors, such as Sprite.
Sticking with the league is probably a good call as it feels less manipulative and is something a gamer could really take pride in seeing it the first time.