I know that Bc4 is supposed to be a "bad" move early in the Sicilian, because Black can nullify it by just playing e6:
[FEN ""] 1.e4 c5 2.Nf3 d6 (2...Nc6 3.Bc4? e6) 3.Bc4? e6
Why is it considered a mistake in these positions, but it's nearly always played in the Smith-Morra Gambit?