It's a bad habit to have 2 versions instead of one fully responsive one, but this is a question for another debate which has seen lots of discussion already.

But somehow sites (and again, I include Facebook as a prominent example) don't sniff the other way around when a user comes to their m.* domain from a desktop browser and don't redirect them to the canonical domain.

That results in people browsing these sites from mobile, sharing m.* links which people then open in their desktops and see a very weird mobile-oriented display.

My question is why do they not do that detection? is it pure negligence or is there something else behind it?