Surely there's a good argument for non-discriminatory digital payments too, given how much of an advantage easier payments confers.

I mean, bitcoin is a thing.

But who ever said anything about an advantage? The argument about free speech is that censorship is bad, so there should be some minimal level of speech that anyone can produce without worry. This comes down to "the government can't regulate me". Some people broaden this to a moral imperative of "we shouldn't needlessly censor", which is a fine moral imperative.

However, moral imperatives are not things we should legislate as they aren't universal, and "the government cannot regulate me" is a far cry from what some are calling for now, which is " the government must regulate you".

Thank you for mentioning the post office. The post office is a great example of a service that could just as well be private and should, in that case, still not be allowed to discriminate.

Please remember the www is international, "the post office" is highly ambiguous.

Also, just because something could be private doesn't mean it should be run for privatised gain. I'm pretty sure the UK post office was recently privatised.

And indeed a privatized USPS could potentially be considered a common carrier and regulated as such, although note that currently neither UPS or FedEx is.

But that has no bearing on financial services (like square or visa, which are all contract carriers) or hosting companies (also contract carriers), or other things like that. ISPs are the one exception which might fit the bill of a common carrier.

The important difference is that an attempt to speak at unwilling subjects via the mail would constitute harassment with common carriers. They're not broadcast mediums since they are point to point.

Yep, I'm not actually sure if there are any examples of common carriers that aren't point to point. Radio isn't a common carrier, you have to be federally licensed and its highly regulated (and for example all of these youtube videos that we're talking about would result in the makers being banned and fined).