"Surely drinking water must be allowable."
No it is not, wolf.
I think, AOG, the main reason is that, by and large, the activities undertaken by Christians during their festivals are not life-threatening. (Yes I know, liver damage, drink driving, etc. are problems, but most of the population are sensible enough to know their limits).
With the Ramadan fasting, especially at this time of year, the practice is positively dangerous for all concerned. In London today it is more than 16 hours from sunrise to sunset. To go this length of time, especially with the current temperatures, with no fluid is dangerous and bordering on the insane. However, no amount of advice on leaflets will deter Muslims from this ridiculous practice. I have no particular concern for them. If they are daft enough to do this then that's for them to decide. However no accommodation should be made for this ridiculous practice and I'm not too sure they should be allowed to drive or operate machinery whilst severely dehydrated.