Why does FTL imply time travel?

No.11725252 ViewReplyOriginalReport
If it's year 1 in Planet A, and year 1 in Planet B, and then someone uses instant FTL from A to B, they will arrive in Planet B at year 1 too. And then they can go back to Planet A again, not breaking any causality, since the time in Planet A is progressing at normal while they are spending time in Planet B. Right? If not, why?