There's no law against selling items at a higher price and doing loads of advertising with carefully worded phrases to make people think it's cheaper.
If it were, there would be hardly any adverts, or companies, left!
I have a colleague who always checks both trainline and the TOC site. Every time, the TOC is cheaper because there are no booking fees, but my colleague continues to check trainline because he is convinced they have access to some extra-cheap tickets. He refuses to believe they are the exact same tickets from the same database, even though he's never found cheaper tickets on trainline...
This is a widespread belief. either people mistakenly believe that TTL has a special agreement with rail companies to sell their tickets more cheaply, or they mistakenly believe that TTL buys up the cheapest tickets and the sells them on.
To be fair, many people believe the same about other travel websites, hence the multitude of ads you see about cheap flights and cheap holidays. As with the TTL, they never seem to claim that they are the cheapest, as there would be no question about that being false advertising.