user23699: 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?

user9684: Bc4 is a good move in every Sicilian. As with all aggressive moves, it provides a target, specifically for a rook on c8, a queenside expansion by b5, and the center pawn fork.  The gambit requires active pieces and quick development  for the pawn, so not only is Bc4 a good move, but it's necessary.

user13012: Bc4 is a reasonable move in most Sicilians. The drawback is that the Bishop might find itself blocked out of the action after ..e6, and the likelihood of that is different in different variations. For example, in the Dragon, Black always plays ..g6 and playing ..e6 as well would leave weaknesses on the Black squares. In lines where ..e6 is played, White usually tries either to sac on e6 or to advance f5. Both of these plans are risky, so White has to be "in the mood".
In the Morra, White has already given a Pawn, and these plans are consistent followups.

23699: <p>I know that Bc4 is supposed to be a &quot;bad&quot; move early in the Sicilian, because Black can nullify it by just playing e6:</p>
<pre><code>[FEN &quot;&quot;]
1.e4 c5 2.Nf3 d6 (2...Nc6 3.Bc4? e6) 3.Bc4? e6
</code></pre>
<p>Why is it considered a mistake in these positions, but it's nearly always played in the Smith-Morra Gambit?</p>
 9684: <p>Bc4 is a good move in every Sicilian. As with all aggressive moves, it provides a target, specifically for a rook on c8, a queenside expansion by b5, and the center pawn fork.  The gambit requires active pieces and quick development  for the pawn, so not only is Bc4 a good move, but it's necessary.</p>
 13012: <p>Bc4 is a reasonable move in most Sicilians. The drawback is that the Bishop might find itself blocked out of the action after ..e6, and the likelihood of that is different in different variations. For example, in the Dragon, Black always plays ..g6 and playing ..e6 as well would leave weaknesses on the Black squares. In lines where ..e6 is played, White usually tries either to sac on e6 or to advance f5. Both of these plans are risky, so White has to be &quot;in the mood&quot;.
In the Morra, White has already given a Pawn, and these plans are consistent followups.</p>
