Although he did not do the experiment himself, he had come up with the idea of what to do and how to do it. I think that only Rutherford receiving credit is okay as the assistants did not do anything to contribute to the actual idea of the experiment.
For example, we credit the invention of the iPhone to Steve Jobs, the engineers of Apple, the designers, etc. - those with actual contributions to the invention and the ideas and not the Chinese factory workers - those who construct the iPhone itself.
