Possible reasons are that sprinklers reduce water pressure for other users. Not true in my experience. If someone else on the plot turns on a tap then my sprinklers stops. Some evenings last summer it was not possible to use a sprinkler as there was insufficient pressure as watering cans were being filled and hose pipes used to water withering veg.
At some sites sprinklers were left on all day I've heard. Why then punish all rather than targeting anyone abusing the system. Why not initially ask allotment holders not to do this otherwise the use of sprinklers will be banned? How will any of these policies be policed? What happens if allotment holders still leave sprinklers sprinkling all day?