McCullum has been pilloried in the media Test for running out Muralitharan when the careless tailender left his crease while the ball was in play to congratulate Kumar Sangakkara on scoring a century.
The dismissal sparked an ethical debate, with the Sri Lankans and several commentators saying it breached the spirit of the game.
McCullum said he was shocked by the widespread adverse reaction to what was a legitimate dismissal that ended Sri Lanka's second innings and left New Zealand a 119-run target to win which they achieved for the loss of five wickets.
"I didn't expect the repercussions from it, it's been pretty hard to swallow," McCullum said.
"I never thought it the wrong thing to do, the rules are there and you can't reward stupidity," McCullum said.
New Zealand Cricket chief executive Martin Snedden said South African umpire Brian Jerling could have approached captain Stephen Fleming and asked if they wanted to proceed with the dismissal but he did not.
"The umpires take the same view as us," he told Radio Sport.
"It was a lapse of concentration on Murali's part, it wasn't a marginal situation and it was out."
New Zealand batsman Nathan Astle was also adamant McCullum was beyond reproach.
"We're fully behind Baz, he was definitely out. I've been surprised by the reaction -- it's totally unjustified."
A stunned Sangakkara was quick to point out that he had declined to run out Astle in similar circumstances when the two teams met in the recent Champions Trophy tournament.
"I could have broken the stumps but I knew there wasn't a run being attempted," Sangakkara said.
"I said jokingly 'you should know better than that'. It was all in fun, it would have been pretty absurd of me to break the stumps."
However, the Test here was no laughing matter and Sangakkara, while acknowledging Muralitharan was at fault, was still disappointed at the Black Caps reaction.
"Players take it for granted, once the ball's gone you're allowed a certain leeway when it comes to courtesies extended between sides," he said.
"Murali was out but it's a case of how you want to interpret the rules. A lot is said about match referees and umpires being the guardians of the game but the players have a lot of responsibility as well."