What you'd like to think is completely irrelevant though. As I'm sure you know, there is a tick box which states something like 'I have read and agree to all of the terms & conditions'. In ticking that box you are agreeing to use the ticket in accordance with those terms & conditions, no matter how draconian or ridiculous they may be.
Yes, but the point seems to be, as Westdene Seagull has pointed out, is that there's nothing in the T&C that says you can't leave the train early.
What you're agreeing to is the right for SWT to implement any fines they seem fit - that doesn't sound like a fair process to me. Nor, does it seem, one that has any force in law and relies on people meekly complying with daylight robbery.