Novikov's self-consistency principle implies that we can change the past as long as we don't know it's been changed. (Well, maybe more accurately, the time traveller has "always" done whatever it was he did in the past.) So even if causality is a requirement, it isn't necessarily violated.
Likewise, lots of time travel in fiction actually has the hidden assumption of a second time dimension. If a second time dimension existed, causality could be maintained along that dimension while seemingly broken in the time dimension we know and love.
And there's always the possibility that causality isn't a given and is simply how we experience the universe. Just because it has always been observed and it makes intuitive sense doesn't mean it is always true. :)
Likewise, lots of time travel in fiction actually has the hidden assumption of a second time dimension. If a second time dimension existed, causality could be maintained along that dimension while seemingly broken in the time dimension we know and love.
And there's always the possibility that causality isn't a given and is simply how we experience the universe. Just because it has always been observed and it makes intuitive sense doesn't mean it is always true. :)