King's Indian Defense ...c5!?

I was wondering why it is that the main line of the King's Indian Defense opts for black to play e5 (1. d4 Nf6 2. c4 g6 3. Nc3 Bg7 4. e4 d6 5. Nf3 0-0 6. Be2 e5). It seems to me that if the opponent decides the petrosian system (7. d5 followed by Bg5 and Nd2), Black can have a difficult game compared to White.

Instead, however, it seems to me that a c5 instead of e5 avoids this problem and may bring some White players out of book. Is there a drawback behind c5 or is the g7 bishop just in a better position than with e5?

well a c5 break is more for benoni type players. it goes for the queenside more and the whole point of the king's indian defense is to launch a kingside attack and undermine the center pawns. 

i think e5 is pushed because it gives black kingside space and protects blacks king while controlling squares on white's kingside. the king's indian defense is all about attacking it seems to me. the drawback of c5 would be having a weaker attack and no kingside space. the sicilian dragon is what you showed which is busted according to a lot of people

 Sicilian Dragon? The Sicilian is a defense to 1. e4, not 1. d4. Also, the Sicilian Dragon is not busted. New theory on the Yugoslav Attack is constantly supporting it by GMs on the Black side.

 If that is the case, that the Black player is looking for a Benoni structure, wouldn't this route seem more favorable, with white's advance, Black avoids the backward d-pawn and furthermore has just as good control over the dark squares.

 The Benoni may be harder to play, but in this position it seems that Black (if he likes Benoni setups) has achieved everything he wanted out of the opening. He has no notable weaknesses. (All this assuming white advances his d-pawn).

In the KID Black usually plays e5 and later Nc6 provoking d5. He wants the center pawns fixed so that he can start an attack on the kingside.

Black sucessfully plays c5 in the King's Indian against the Sämisch variation (5.f3), though.

As others have already said, c5 is not at all bad per se, but after d5 will mostly transpose into something Benoni.

its still a sicilian dragon setup

with a possible moroczy bind for white. so i guess..... you're wrong

right, so lets say in the OP 7.Bf4 or Bg5 then cxd Nxd

i see what you mean about 7. Bf4 but i'm not sure the other variations are forced

I guess it's just a matter of taste whether you choose c5 or e5 first. Probably most KID players will go for e5 because, like somebody already mentioned, it will give good kingside attack chances for black - that's why they usually play KID in the first place, right? In contrast after C5 black's play will be focused on the queenside, but that's not saying his chances would be worse than after e5. Just follow your own preference and play situations you are comfortable with.

Chess is not an easy game, and it does not always matter what "seems" to me, and you. Both sides can have a "difficult game" after 6...e5, and the main reason is not the quiet 7.d5.

For the record, after 6...c5 7.0-0 Black will either take on d4, when it's a Maroczy Sicilian (solid but rather passive for Black) and after 7.d5 a Benoni.

Is there a problem if Black does not capture after 7. 0-0? After 6. ...c5, control of the d4 square looks to be a credible idea.

After 6...c5 7d5 e6 after a later pxp white doesn't have to reply c4xd5 with a unbalanced modern benoni structure but can play e4xd5. So the move order isn't perfect. Black can try delaying exd5, but it's quite a difficult game for black too.