Rules are also often created to prevent extreme behaviours that are unlikely to exist, e.g.
Imagine starting each day with a 20km descent (Bruce could do this if he got a lift to the top of Mount Lofty each morning, he isn't though). Or Steve getting lifts to the top of Yad Moss and descending down to Middleton-In-Teesdale, lather, rinse and repeat for a free load of miles each day. Or Kajsa basing herself in Bedoin and using Mont Ventoux...
Rather than say "Don't take the piss with getting lifts to the tops of big hills over and over again" a rule of "You must start where the previous ride stopped" is formulated, which makes it a logistical nightmare for people who just wanted to move about the country without abusing hills. It also, handily for Guinness, stops people chasing the same tailwind each day.
"No drafting" might have come around from the fear that a rider will sit in the middle of a huge pack of faster riders and save ~40% of energy and nullify all head/cross-winds. It's very unlikely anyone would be able to arrange this day in day out, but it also penalises people who just want to sit behind someone's wheel for a bit when riding with company and taking it in turns to be at the front.
The UMCA have provision to add new rules (which won't apply retrospectively) so if they spot something which isn't in the spirit of the challenge (as they are looking at the rides every few days) they can look to add a rule to prevent it, or at least suggest that it stop. Guinness seem to set their rules out in advance, and if they're not monitoring everything day to day they won't know about something until the end.