At same time you suggest this, someone else is yelling at me for not taking up the cause of the fans against the team because, darn it, they must must must must must must must sign Bryce Harper. Let's take a moment to consider the different disciplines in sportswriting. There are beat writers. There are columnists. Columnist are paid to offer their opinion -- and yes make an argument that could sway fans. In the end, it's always the fans who get to decide what they have as an opinion. No one is forcing you to accept the argument/opinion presented by a columnist. Columnists are freer to do this -- to share an opinion, to make a case, to present an argument. Beat writers are bound by a different role. Beat writers report on the actions of their beat -- and they attempt to explain what the beat is thinking, or, in my case, why the Cardinals are doing things and what the Cardinals are doing. Beat writers are not advocates for these reasons, they are conduits so that fans have the information to, as you suggest, decide for yourself. Beat writers serve that role. Beat writers can also write pieces that are aggressively analytical when reporting supports those stances. I think of them as theses papers. I have to prove my theses with reporting that I have to verify and attribute -- and that I am accountable for.
So, yes, in some cases sportswriters are paid to influence the fan base. That is their duty because that is their job. In other cases sportswriters, like beat writers, report and report and report and the fans can decide what they want to do with their dollars or minds or whatever.