Because it isn’t the first known instance of the character state “fin”. By the definition we’re using here, a character is out of place iff the first known instance of its derived state precedes the first known instance of its primitive state. Later instances are not relevant. Your example would work if no other finned fish fossils were known.
Now of course there’s a problem with @scd’s claims. No single fossil can falsify evolution; one must consider the fossil record as a whole, and one anomaly doesn’t destroy the structure. Not even a Precambrian rabbit.