Have researchers any incentive to publish negative or confirmatory results?
There is certainly incentive to publish negative results: a good negative result that clearly establishes, "X is not possible" or "X is not true" can be extremely valuable.  I've published some negative results myself, and am proud of the work.

Negative results, however, typically must hew to a much higher standard of evidence than positive results, in order to distinguish between "X is not possible" and "I can't figure out how to do X." For positive results, showing "I can do X" automatically implies "X is possible," but the syllogism does not hold in reverse.  I think this is one of the main reasons why publishing negative results is so difficult. Also, it's easier for reviewers to argue with negative results, and harder to argue with positive: with negative, it matters strongly why they are negative, while with positive the "why" can be relegated to discussion and hypotheses for future investigation.

Confirmatory results, however, are much harder to justify in absence of an explicit mechanism requiring them, as for medical studies.  Typically, we get confirmatory results not directly, but indirectly through the development of new results building on the prior results: the prior results get confirmed through their use in the controls in the new study.