Technically, it is in season, with the appropriate weather... it's just that said weather can't happen naturally.
Don't get me wrong. I do think this is an oversight, and it could/should be changed so that nobody requests something that is only technically in season.
Edit: I mean, you wouldn't assume that magic bait (which technically makes out of season fish available) should allow requests for out of season fish. You wouldn't also expect that they require you use a rain totem in winter for a request.