If they were evading, they'd have lied and for a ticket from Lostock Hall or claim to have been spotting/seeing a relative off or something, or even run off. The OP did nothing of the sort. So why on earth does anyone speculate otherwise?
Just to clarify - he is buying onboard a Virgin service between Wigan and Preston, so he doesn't really have any other option but to buy the correct ticket.
I shouldn't go so far as to suggest evasion, but I am guessing that he is choosing to buy tickets on the board in the hope that he doesn't get asked to pay. If he gets asked, then he pays the same onboard as he would have at the station, so there is no incentive to try and buy in advance.
It would be extremely naive to believe that people don't ever do this; I may have even done it myself, in my youth, but I have a bad memory, and that's too long ago to remember for sure!
We have no idea if he does buy a ticket when he gets to Preston, he did not answer that. But I am guessing that it would generally be easier for him to buy his ticket at Wigan.
Ok, maybe I shouldn't speculate, and I admit it is rather rude. However, I do think it looks suspicious, even though it may be entirely innocent.
I can't think of any suitable similar example for this, the best I can come up with is if there was a toll bridge where you could pay at either end. If you usually left it until the far end to pay, and knew that it was often unmanned, then I think I would also be suspicous.