"I struggle to see what the big deal is with a different name appearing on your statement. It happens frequently in all areas of life."
We clearly see the world from a very different perspective. It does happen sometimes, but there is usually no ambiguity who the real organisation is. On that ticket purchase I'd argue there is huge ambiguity. If you're trying to suggest that "Express", "Thameslink" or "Southern" all have a separate existence, which of those brands is the successor to South Central? If Amex hadn't sorted this quickly, who would have I raised a dispute with? Answer none of them, in legal terms it has to be GoVia. But I only know that because I read these forums. I'd wager 95% of customers on this route have no idea.
I've spent much of my working life in financial services, an industry that has been shown recently to have some appalling ethical failures (and I do not excuse those scandals).
That said, on the matter of being clear who the client is legally contracting with, it was taken very seriously in financial services and much time and effort was put into getting it right. I'm not a lawyer but I've spent years working alongside them in multiple corporate restructures, mergers, demergers and takeovers. We went to huge trouble to ensure all contracts, marketing material, websites etc had the right name. Yes, we might use a "brand" but everything had to make clear who the legal contracting party is. There'd always be a statement such as "smiley bank" is a brand name of "mega bank", "full registered office address", "company registration number". The lawyers I worked with were unanimous that if we failed to that, and anything went to court, the court would go against us and our contract would not be enforceable.
Not taking these matters seriously (and GoVia and dft are being cavalier with names) is (in my opinion) failing the "treat the customer fairly" test.