It's because "simultaneous" is relative in the same way that "in the same place" is relative. You can bounce a ball against the same spot repeatedly, but if you're on an airplane and another observer isn't, the places the ball hits are separated by a large distance in their reference frame.
Most concretely, the FTL-breaks-causality conclusion comes from: If special relativity holds (it does), and you have FTL travel, and there is no universal privileged frame of reference affecting your FTL drive, then it's possible to get back to your starting point before you leave by making multiple FTL hops. You have to be able to make the two hops in different reference frames, that is, you need to do a bit of conventional acceleration between hop 1 and hop 2.
From everything we know about the universe it seems unlikely that there's a special preferred frame out there, but if for example you're developing a hard-science-fiction setting and want FTL without causality violations, you can throw in that background detail to keep the physicists happy.
> it would be possible to OBSERVE events in the wrong order
True, but of course that's possible even in normal earth-bound newtonian physics. All of the discussions about relativity assume that all the observers are taking speed-of-light delays into account when they're observing.