The difference is that gmail isn't publicly serving your emails to an open audience. The restrictions on Drive are only for publicly-served content.

That isn't true, they already block you from downloading content from private shared folders that they decide to restrict.

I dunno why this is downmodded, given that e.g. OneDrive already applies its rules to all content, not just shared stuff; and Facebook is actively censoring even private chats (it will refuse to send some links, e.g. to certain websites about cannabis).