I think a lot of people don't really understand how these things work and are jumping to conclusions a bit.
Firstly all those bleating on about civil law, how its unenforceable, etc etc. As a few people have mentioned and been completel y ignored the railway system in this country is governed by a specific set of bylaws which dictate what a train operator and cannot do. One of the things these bylaws allow operators to do is issue legally enforceable fines and penalty charge notices, something most private companies cannot do.
Next, lets talk a bit about terms and conditions on ticketing and the reasons why they exist.
The primary reason for lots of different types of tickets is that it is a very effective method of demand and capacity management.
Imagine a train travels from A to C, and stops at B. Lots and lots of people travel from A to B, but hardly anyone travels to C. This results in an overcrowded train from A to B and a virtually empty train from B to C. One of the best ways of dealing with this problem (Obviously on rypt-rail he would simply order 9 billion new coaches, this is because rypt knows everything (Except where he doesnt, then he runs away and hides like in yesterdays 777 thread)) is to have different tickets available for the different trips. By increasing the price of a ticket from A to B it encourages less people to use that train for the A to B section of the journey. By lowering the price of the ticket from A to C it means that the train is no longer under utilised from B to C. The airport example is another good one. If you've got a city to city service with an airport in the middle its no good if nobody going from city to city can't get on the train as its crammed full of people only going as far as the airport.
Clearly if you didn't have easily enforceable rules, everyone would simply buy a return from A to C, get off at B, and the overcrowding issue would continue.
An absolutely perfect example of this are trains out of London to Birmingham/Manchester which also stop at Milton Keynes. Milton Keynes is within the commuter area of London but the trains on the Birmingham/Manchester route are long distance Intercity services not commuter trains. Therefore these services are pick up only at Milton Keynes - you cannot get off. This stops them being filled to the brim with people going to and from work in London, which would discourage the use of the train by people making long distance trips - the very purpose for which they were designed. Get off at MK, get fined. I'm sure you'd all cry about that as well.
You will all now argue that Eastleigh is so close to Southampton that its unlikely there would be a huge amount of people wishing to travel to Eastleigh and nobody wanting to go to Southampton. This is probably true but the thing with rules is that you can't have 900,000,000 different variants, its far more workable just to say 'You must complete the journey only for which your ticket is valid.
Advance Purchase tickets are not a right. They are a privilge the train companies use to offer incentives to travel on services which are less frequently used.
It is very clear that these people were attempting to get around the rules to get a cheaper journey. This is why they asked for a ticket to Southampton, rather than Eastleigh. Whilst I agree on the surface its quite harsh that they were punished, it is also fair and just - you cant have one rule for some and one rule for everyone else. You have to apply restrictions equally, or they become unfair. The ticketing system does need revision and in some cases is quite convuluted but until it is revised, abide by the rules or use your car.