Despite being way outside their key demographic, I'm not cool enough to pretend I don't read newspapers or watch TV or browse the internet to make a smart-arse lie that I don't know who this guy is or what global boy-band he's in. (I've always wondered; how do some people manage to spend so much time on a showbiz oriented website, especially the music specific part of it, yet be completely ignorant of popular music culture?

Must be the people who claim they never listen to anything in the Top 40.)
My thought is, what's the legal obligation to the fans in this situation? By which I mean when a band member leaves part way through a tour, but the tour continues? People have paid handsomely to see them perform on the basis and understanding that they'll see the
whole band perform. (It's telling that the band see no reason to cancel any gigs as it's 'only' Zayn; would they do that if it was Harry that had been signed off? (Get me, I know his name. Again, I'm not cool enough to pretend I've never heard of him.))
Obviously the legal position will depend on whichever country/ies the tour is currently in, and I'm assuming no-one will be au-fait with those, but what if this was the UK? Is there any legal 'comeback' from the fans, promoters etc in this sort of situation? OOH, the band is still fulfilling it's obligation to put on shows, but OTO, 1D is made up of five singers, each of which is (presumably) equally prominent, and the shows were sold and promoted on that basis.