Jay Rosen, professor of journalism at New York University, agreed that "this is not a new problem", but said it had got worse.
"There have always been sources that tried to win these terms, and lately more and more have succeeded. What was new and significant in the Times story was that quote approval is now the norm for a whole layer of campaign sources; most of the reporters working the beat had already come to terms with that, the Times suggested."
Rosen said that reporters told him that the process has been building for years under George Bush and now Barack Obama. It has been allowed to happen, he said, because of "what academics call the collective action problem".
"The press can behave as a crowd of independent minds, but it has problems acting as an united front on anything," he said.
Asked what it would take to reverse the trend of quote approval, Rosen said: "A press willing to say: 'Fine: we'll report the story around you. We will build it from the outside in.'