They find anecdotal cases where a campaign contribution appears to have influenced a politician's vote. Academics on the other hand take a more systematic approach. They look not for anecdotes, but valid generalizations. Even if we find irrefutable evidence that one person's vote is bought, we may not be able to generalize to politicians as a whole. Using this more systematic approach, academics tend to find less evidence for vote buying.
Vote buying occurs, but on a very limited scale.
Political scientists differ in two main ways from journalists in how they investigate these questions. First, political scientists look at all the instances, not just a few that seem suspect. They are interested in making a generalization. Second, they are more careful in drawing conclusions about whether votes were bought, i.e., whether money bought a vote or whether there was some other reasonable explanation. This part takes a little more explanation.