Start with the trouble in River City. First you have somebody, Professor Harold Hill, telling a crowd of people "I am upset about this pool table, and you should be too." However, in the musical it is made clear that Harold Hill, who is not a Professor, nor even named Harold Hill, is NOT really upset about pool tables. His purpose is "get the people riled up, so I can sell them band instruments."
In a similar way, there are certainly some people who write, blog, tweet, appear on TV, etc. who have one or more agendas when they write.
Certainly I have an agenda when I write, several agendas
1. To advocate for the bottom 60% in America
2. to refute spin which favors the top 20%
3. to look at the big picture
4. to argue against hate, greed, selfishness, and cruelty and in favor of love, sharing, understanding, reason, and kindness
Now any pundit out there may claim some sort of ideals for themself that they are trying to promote, but they may also be trying to promote themselves - that is, read my articles, buy my books, look at my website, and so on. One primary objective - make me money, give me power and influence.
But it still seems like a fair question when somebody writes "I am upset at Obama, and you should be too" or "I am upset about NSA spying and you should be too" to want to know "what is their motive?" and "what is their agenda?" Or you could find yourself paying a lot of money for a band instrument you did not really want for a band that does not really exist.