> what makes an email different from a web page, other than that it has been sent to you?

I think it's to do with perceived ownership of the environment in which the information is consumed.

Your website... feel free to style, brand, make pretty or ugly. You can make the experience consistent within your realm.

My inbox... I get to control my workflows, how I consume things, in which order, etc. I choose to make the experience consistent within my realm.

That holds fairly well as a definition for why I prefer chat clients that grant me the ability to make all messages consistent, and that do not allow the sender to dictate terms. It also holds up with things like Netflix, it's their realm they can knock themselves out on their design.

It does seem to be whether the environment it is presented is "yours" or "mine".

It's your client, but the site's content, in both web and email cases.

You are free to set your client to ignore styling on the content, but the content should be stylable for everyone else that wants it to look good.

One is push, the other is pull. I can choose never to go to a URL again, but I can't choose to never receive spam from the same person from a different address and server.

Also, I think many people do that with AdBlock, so it's not unheard of to block styling on webpages of dubious origin.