Two years after returning from France to try to make the All Blacks, Williams will head to Japan after the Super Rugby season to take up a lucrative deal with the Panasonic club.
It's then on to Australia to play in the NRL in 2013 to honour what he says is a handshake agreement made several years ago.
"People are going to question his loyalty, to be heading away, and his commitment to New Zealand rugby or to the Chiefs," Rennie said.
"Really, it's his commitment that's taking him away from here.
"He made an agreement on a handshake and he's doing the honourable thing in committing to that. He should be applauded for that as opposed to put down for it."
While Williams didn't named the NRL club he will join, he has been linked with a move to the Sydney Roosters.
A Kiwi league international, Williams switched to rugby after controversially walking out on the Canterbury Bulldogs in mid-season in 2008 and going to France to play for Toulon.
After his return to New Zealand, he had a season with the Crusaders before joining the Chiefs for the 2012 campaign and has started to show he is coming to terms with rugby.
The 26-year-old has been in excellent form for the Chiefs at second five-eighth and was a frontline selection for the All Blacks against Ireland for the June Test series, when he took his tally of caps to 17.
Rennie said he didn't know Williams at all until he met up with him at the Chiefs.
He said had been impressed not only by Williams' efforts on the field, but also his diligence off it and his relationship with the rest of the players.
"There's almost an impression that Sonny is bigger than the team or bigger than the game, but he's very much one of the boys and certainly loved by his teammates."