There's two arguments here, one is the free speech argument, other is public order.

Now personally, I wish we lived in a world where people would just mind their own business (well I do, sucks for earthlings) but unfortunately if you're going on reading about, god I dunno, Nazi Jewish genocide for example on the train, you are likely to set off the busybody sitting next to you who can't help but poke their nose somewhere it doesn't belong, even if I'm reading about Nazi Jewish genocide because I'm against it and I just want to know what my opponents think, doesn't matter to the idiot busibody and they'll start shit on the train, which you don't want.

That said, if they're gonna provide wifi, blocking shit needs to be done carefully otherwise they'll just seem to be ideologues. Personally I'm of the opinion that it's usually leftists that go and complain to the authorities about such things which is why you see generally their opposing side of the political aisle banned (rightists also complain but tend to complain to the person directly rather than complaining to the authorities imho)

That said could also just be a low level employee doing this shit. Always online types tend to be woke and they tend to take the online oriented positions. I've had to deal with such in the past in various cross cultural contexts and it's very tiring explaining to them that there are people who don't hold their opinion and that they can't go about insisting everyone agree with them.