The reason you can't have hay fever is nothing to do with sneezing while flying, plenty of aircrew aren't handling the controls, and not all of those that do are doing warp factor 6 in the low flying system. The main reason is that anything that blocks your nose, ie your sinuses, can be dangerous when flying, because of the changes of pressure. Blocked sinuses wont change pressure as the cabin pressure rises and falls (either in climb/descent in an unpressurised ac or in the event of a de-pressurisation in a pressurised ac). Sinus pain can be immediately, painfully disabling. So it is a large risk, taking on someone who might be prone to regular periods of blocked sinuses. Also remember that we fly all over the world, so it is not a just summer issue.
However, modern medicines such as loratadine are very effective at stopping the symptoms, so once money has been spent on training you, it is more effective to dose you with something effective that waste the millions by chopping you. You might infer from that that if you take loratadine as a precautionary measure (and it is available from any good supermarket) you won't suffer from hay fever .......