I believe my husband heard on our local Monterey Bay NPR affiliate (which serves the tomato-growing Salinas Valley) that when the stem is removed from the tomato, a porous area is exposed at the crown of the fruit where salmonella contamination can enter the flesh of the tomato via the water in which the harvested tomato is washed. Hence tomatoes on the vine are safe, as there is no exposed flesh.

As for cherry tomatoes, I dunno -- maybe the stem area is so tiny that there's not enough exposed area to allow significant contamination?

Claire