The Daily Stag Hunt Staff recently ran across an article suggesting that the media are going easy on President Obama. (Real Clear Politics).
The political views of the observer probably have a lot to do with the observation in this area. Nevertheless, a quick internet search also turned up an article arguing that the media went to easy on George W. Bush.
There are three possible alternatives. (1) The newspapermen represent the few who are secret partisans for both Presidents Obama and Bush. (2) People who did not like President Bush said the media went too easy on him and people who do not like President Obama say the media is too easy on him. (3) An incumbent President has a lot of ability to control their own media coverage.
The first alternative, of course, is ridiculous. The other two are both likely in part true. Obviously, perspective on what is true plays a huge role in the perception of media bias. The third point is nontrivial, though; a sitting President is a good media "catch" and can use his value as a bargaining chip for nicer coverage. For example, periodically a president may wish to give an exclusive interview - and if your network is perceived as being "unfair," he's certainly going to give that interview to another network. Journalists that "play ball" may get better access, more information, and nicer treatment. This does not have to be done out of evil intent or out of an explicit desire to manipulate the press (although it is always possible); this sort of relationship would naturally develop between any group of individuals that have to interact every day for at least four years where each side wants something from the other.
In a sense, the white house reporters are all in a "market" for information and the President can choose to give that information to the person that offers the best "price." A nice interview is "less costly" than a hardball interview. Since there is only one President, he has a greater advantage in this relationship.
For example, there was a dustup in 2007 between the Bush administration and NPR, where the Bush administration wanted to pick the reporter for an interview. (Link here). NPR wanted to pick the reporter, so they refused to accept the White House conditions. Instead of giving in, President Bush gave an interview on the subject to Fox News instead.
There are also reports of the Obama administration punishing reporters for not playing along. (Link.)
So, while partisanship and bias likely play an important role, the structure of the relationship between any president and the media gives the president some amount of control over his (and perhaps someday, her) coverage that other politicians do not have.