I introduced a nut free policy at a site which had previously allowed nuts. There was a brief, signage etc done. No nuts were found on audits. One incident occurred where a pack of crunchy nut cornflakes was found in a kitchen by chance (but these were well out of date and so presumably had been brought in prior to the ban and were disposed of.) Additional man hours required? Probably 10 hours total as a generous estimate and then it was added onto audits which were already being done so no ongoing cost.
Yes the risk is small but I suggest you think about that lovely legal phrase "reasonably practicable". Is it reasonably practicable to ban nuts? Yes. It's cheap and effective; far more so I would argue than just relying on people washing hands.
And yes, no-one knows what the risks are if they're honest. But it's a simple thought process for me of consequences, probability and cost. The consequences could be death. The probability is low (but not impossible) and the cost is low. Therefore, for me, I would ban the nuts because of the consequences and cost, despite the likelihood being low.
I often cite this but in the Cadbury's Salmonellae case, they thought that as no case had been proven with <10 cfug-1 of Salmonellae, <10cfug-1 did not cause disease. They were wrong and fined a lot of money as a result. I'm just thinking through due diligence; if you could take a step that's cheap and reasonable to prevent an issue (even if it's already low risk), why the devil wouldn't you? It strikes me it's more about proving a point than really assessing the risk. In life we all have to accept things being a bit imposed on us even when we believe the risks to be minimal. The risks are minimal that someone could cause an illness by handling cases of product (rather than open product) but I still tell people to wash their hands before going into packed product areas.
I once got told by a machine manufacturer that a particular part of a machine "never broke". 5 breakages later in the space of a month they were still telling me this part would "never break". "Never" is not a word in food safety IMO, it's just controlling different levels of risk and by banning nuts altogether you reduce the risks a little bit more.