Earlier quoted context omitted.
> That's not really how it works. People work on things they're interested in, not what they get assigned to. You have no idea how management and funding of large-scale research projects works.
At the level of picking a field, people absolutely work on what they are interested in (and have expertise in). You're not going to get particle physicists working on organic chemistry just because you've "allocated" them there (and they wouldn't be trained in it anyway).
If I consider what kinds of crappy jobs some excellent physicists who could not find a position in research had to take, I am not so sure.
The truth in my opinion thus rather is: Because, as you wrote, "they wouldn't be trained in it anyway", they won't be allocated for "organic chemistry" in your example.
On the other hand, if they were, I really believe that they would prefer this job over the one that they had to take.