The media writes about these guys. Most players don't want to get on their bad side because they're just like anybody else. If they don't like you, it's easier for them to write bad stuff about you or sway things negatively. If you're a great player and have good relations with the media, they're less likely to throw you under the bus. I assume he doesn't want them writing about steroid stuff all the time throughout his career, especially after he does so well. If the writers see him in a negative light they're much more likely to to pound this into the ground instead of dropping it like they did with Peppers.
As far as the fanbase goes, we go to the games, we buy the merchandise. His contract with the team isn't determined by what we think, you're right about that. But there's merchandis to be sold, jerseys to be sold, radio shows to do, commercials to do, product placement, etc etc. That additional income. But besides that, its just not about money. I assume most players would rather be viewed positively by fans than negatively. He doesn't want to alienate the fanbase.