This seems like complaining "rich people pay for something, that must be wrong"? Are you also against the owners of a Bentley driving it on the roads, because the rest of us drive Fords or Toyotas?
It's no different from any other charter train, just that this one is extra fancy and expensive. By paying track access charges it'll contribute into the pot of running the railway. That helps to maintain the track etc for the rest of us. There's no public money being spent, if anything it's helping put more into the public pot. I'm not sure how that's too difficult to understand?