It's everything at once, and that's why i love it.
A) it's to disguise his voice
B) it's because he becomes a bat, an animal, once he's in the suit whether he realizes it or not. it's how he unleashes his rage.
C) he's doing the voice but the cowl seems to be enhancing the volume or distortion a little bit to help with the disguise and/or help him come off more like a beast
Somebody said that they enjoy it because Batman is a guy who never grew up. What he's doing is a little bit crazy and not exactly a normal adult thing to do. He doesn't care what he sounds like, he's just going for it. The voice is as over the top as his costume, as the entire concept of "the batman".
I would have to agree.
Also, ive noticed in the trilogy, where he's clearly in mission mode, his brain doesn't let him turn the voice off. He just IS Batman, when talking to Lucius on his way to take out Joker. Or talking to Catwoman when they're on their way to the batpod. But when Batman is at his most vulnerable point, he talks normal. No growl at all. Like when he's in the batcave talking to Rachel when she's drugged. He then takes his mask off and you see how much he knows he can't be with her (in that moment). Or when he's stabbed by Talia. He talks normal during that scene AND when he's speaking to Catwoman while trying to take the rope off his neck, still wounded. But once he enters the bat, he can't let pain or anything get in his way. He's 100 percent in Batman mode trying to get rid of the bomb. That's when the growl returns.
I love it. I dont care what people say. I get why some may understand all of this but they simply didnt like the execution of it. The sound of the voice. The delivery. But i dont think it's as bad as people say. I could at least understand every word coming out of his mouth, and even if it's a little over the top or whacky at times, i think that fits the entire idea of the character. So i embrace it.
There's a good argument in saying the voice should be calm because he's a ninja and bats aren't growling all over the place. And that's why Keaton's approach works. But there's another good argument if you say "well, if that were true then he should have a super high screeching voice

hehe

since that's what bats sound like". Then you have the fact that he's not trying to come off like a normal human being, so the idea of him talking normally or whispering would not work well with how he sees himself. Making it seem like he's demonic, other wordly actually goes well with Bale's voice since no normal person speaks that way.