I'd guess that's to stop fun and games such as being driven to the top of Venotux (or some other big hill) for the start of each day for some free miles of easy descending.
The HAMR rules don't prevent this either, nor do they prevent transits mid ride (as long as the tracker is turned off during the transit). Imagine being dropped off at the top of Yad Moss and freewheeling most of the way down to Middleton-In-Teesdale (yes, there are some up bits along the way), then picked up by vehicle and taken back to the top of Yad Moss. A back of an envelope calculation gives 250 miles available in less than 14 hours, and any eating can be done in the van on the way back up (and not much calories or rest would be required given the relative lack of effort required). It's not that much faster than proper riding, but it certainly would be easier on the body. Horrendously tedious though, and far from the spirit of the original record. Good to see no-one, so far, has attempted anything like this under the HAMR rules. (I think they'd quickly add some kind of rule to prevent this if someone tried it.)
The Guiness rules seem more the Audax spirit if anything (I know several Audaxers who had mechanicals and received lifts from strangers [lifts to bike shops or houses for repairs] and had to be dropped or ride back to where they had originally stopped before continuing).