It seems this easement is used by more people than I thought. I have dealt with many revenue activities on both the routes via Purley and the affected stretch of the East Grinstead lines, and very very rarely have I actually come across anyone using this easement myself.
The issue is that it flies in the face of how point-to-point (ie. non-zonal) tickets are generally meant to work, and even works against the legislation which was originally designed to counter people using point-to-point tickets further than their validity. Take for example, the bit in the Railways Clauses Consolidation Act 1845, Section 103, which states:
"... if any person knowingly and wilfully refuse or neglect, on arriving at the point to which he has paid his fare, to quit such carriage, every such person shall for every such offence forfeit a sum not exceeding level 1 on the standard scale."
http://www.legislation.gov.uk/ukpga/Vict/8-9/20/section/103
You can't claim that a ticket from London to Purley Oaks being additionally valid to Chipstead - invisibly! - is something which helps that cause.
Quite how railway employees are meant to determine if somebody has actually done something wrong, when it's not even possible to immediately tell whether you have actually paid a fare to travel to that point - or another, or both - is something which needs to improved upon. There are several of these rules around the Southern operating area, and not all of them are to be found in a universally accessible format or in regular briefings. Staff who have drawn a short straw with training (or passengers who are at all interested in these matters) have to do things like rely on word-of-mouth, printouts from ticket offices etc.
Funnily enough, there was actually a briefing about the subject of this thread early in December, but the fact is that it was still buried in an email attachment in the list of all the other emails, with the relevant information being several pages into it, and not accessible on the tablets which are used for ticket sales. Ticket offices are simply more likely to know on the grounds that they are local, and that literally is the main focus of their job - even an OBS, who is mainly there for customer service, has other things on their mind and will also constantly switch between areas of the country.
(Given the local state of affairs with regards local ticketless travel, I wouldn't be at all surprised if most people
outside these forums who end up buying tickets to the "wrong" stations think they're being clever by short-faring, and actually end up paying just the same price as if they'd got a ticket another couple of stops to their actual destination. But there we go.)
