At that point they're getting negative value out of tabs - people can't set different tab widths and also can't use editors that don't use a width of 8 (or need to reconfigure editors that have a different default). They'd be better off just using spaces.
No, they aren't (but you're free to disagree) and any editor worth their salt can alter how many spaces a tab is displayed with.
I was going to say you're obviously incorrect because in what editor can't you change your tab width? But I realized you are probably talking about the kernel.org project. Is that what you mean? There is an agreement within the Linux kernel project?