He didn't walk out. He played all six games in the match and lost the match in the last game after he played a opening that was know as losing by everyone after a piece sacrifice (it was speculated that Kasparov was gambling that deep blue wouldn't make the sacrifice and then he will have the upper hand).
Deep blue played pretty much "like a machine" except for a few moves that where considered very human because they hold very long term advantages.
After the match Kasparov hinted that IBM must have cheated, of course now a days a machine running in a off the shell desktop machine will surely beat the current world champion in a match, so Kasparov was just being a sore loser at the time.
He felt that the engineers intervened DURING the games, and obviously did not feel that intervention between games was reasonable either. The former was disallowed, and the latter was taken advantage of more than Kasparov thought it would be.
He wasn't being a sore loser and there was a reasonable amount of agreement among other chess professionals (engine authors and players) that his claims were true.
Keep in mind as well that when he asked for a rematch with more stringent scrutiny in place, IBM declined and nearly immediately dismantled Deep Blue.
The unfortunate thing is that kasparov would ALWAYS have looked like a 'sore loser' unless IBM decided to implicate themselves of wrongdoing. Kasparov being silent also does a disservice to the community, particularly since he was neither the first or only person to point out the inconsistencies in Deep Blue's play.
Keep in mind as well that the 'real circumstances' make it so both parties responses were rationally founded within the context of the available information at the time.
We will never know for sure. If I was IBM, after the cheating accusations I'd probably refuse a rematch, too, even if I didn't cheat.
Why? Because unlike before, it would generate bad publicity for me now (the reason for a rematch would keep on being mentioned), and God forbid if ubermotivated Kasparov managed to triumph this time, under tighter scrutiny. Now the public is convinced that I cheated before... The atmosphere was spoiled and they had nothing to win in terms of PR anymore, really.
It was only reasonable to swipe it under the rug and move on. Obviously it does not mean that they were clean. Dropping Deep Blue is not an indicator of either in and of itself, because it makes perfect sense in either case. We just can't know.
My point about the rematch is that it made more reasonable to the chess community to believe Kasparov's claims.
From IBM's perspective, it made sense as you pointed out. From Kasparov's perspective, it also made sense for them to decline , but for a wholly different reason.
It really was just a sad situation where nobody could come out on top except for IBM. Even if they DID cheat, they were the only ones who were capable of proving that. IBM simply outsmarted Kasparov outside the game.
The reason Kasparov hinted at cheating was that he had the measure of Deep Blue. When it played the random move it did something that fell outside of what he had learned to except. Hence, either IBM cheated by making a move for the computer or it was showing human intuition. It was pure coincidence that the move was actually better than what it's algorithm would have chosen but Kasparov wasn't to know that.
It broke his composure and concentration and the match was lost.
When I think of Deep Blue not playing like a computer, I think more so of it leaving itself open to a forced draw the one game when it had the upper hand.
Deep blue played pretty much "like a machine" except for a few moves that where considered very human because they hold very long term advantages.
After the match Kasparov hinted that IBM must have cheated, of course now a days a machine running in a off the shell desktop machine will surely beat the current world champion in a match, so Kasparov was just being a sore loser at the time.