[spydog]

I check/call the turn/river against an unknown.

He will likely bluff at the turn with a 4-flush. If you check-call the turn then he will fire again on a non-heart river because your play will look like a busted flush draw. This is very profitable for you because you are getting him to make 2 mistakes with a worse hand.

If he's on air then he is stabbing at the turn and river for the same reasons.

On the flip side, your turn bet allows your opponent to make the following good decisions:

1) Call with a draw that they can fold on the river.
2) Raise with trips.
3) Raise a flush.
4) Fold a crap hand.