The 28-year-old Williams was a late inclusion in Stephen Kearney's world champion Kiwis squad, forcing the coach to drop Melbourne forward Tohu Harris.
Williams, who won Australia's National Rugby League title with the Sydney Roosters on Sunday, had ruled himself out of the World Cup in order to take a holiday before he changed his mind.
"The axing of Harris from the Kiwis World Cup league squad a day after his selection, for sport's No. 1 narcissist Sonny Bill Williams, is, in my opinion, disgusting," New Zealand Herald sports columnist Chris Rattue said.
"Camp SBW hold the power and wield it disdainfully while weak men fall at their feet.
"This is a day of shame for Kiwis league."
The NZRL was forced on to the back foot when the decision was made to bring Williams into the squad with chief executive Phil Holden stating the organisation had been placed in a difficult position.
"It's been quite challenging because, at the end of the day, whichever decision New Zealand Rugby League made, we were going to have some people saying, 'that's a great decision' and others that are going to go, 'that's not a good decision'," Holden said.
Former Kiwi coach Graham Lowe criticised the handling of the selection issue.
"It shouldn't have got this far actually," Lowe told Television New Zealand. "It's been amateur hour in my view.
"The cold hard light of it all, we need to win the World Cup. And to do that we need to have the best players.
"Sonny Bill is the best player on the planet, the Kiwis couldn't do without him."
Other media pundits agreed the decision was an easy one, pointing out that Harris was likely to be a fringe player while Williams was an automatic first-choice in Kearney's side.
"The Kiwis selectors had little choice but to include him," New Zealand Herald rugby league writer Michael Brown said.
"They were duty-bound to select the best squad that would give them the chance to retain the World Cup they won in 2008."
Williams, who won a rugby union World Cup with the All Blacks in 2011, has refused to sign playing agreements any longer than a year after he walked out on the NRL's Canterbury Bulldogs in 2008 and his future playing options have been hotly debated.
He has been tight-lipped about his plans, but is widely tipped to quit the NRL to revive his rugby union career in New Zealand with Super Rugby's title-holding Chiefs.