The Muslim faith doesn't believe there should be interest charged on borrowings, a practice undertaken by banks like BNZ.
All that is absolutely fine. He is not the first to put his religious beliefs above his chosen sport nor will he be the last.
In the furore that has followed, people have conveniently forgotten that Michael Jones refused to play on Sunday's due to his strong Christian faith. As did North Auckland great Sid Going.
It is hard to imagine that Jones or Going would have been subject to this kind of scrutiny for what is a simple decision from Sonny Bill.
Funnily enough the Christian faith, traditionally, is also against the practice of usury or charging interest on borrowed money.
Conscientious objection isn't even restricted to rugby. Good friends of Williams, South African cricketers Hashim Amla and Imran Tahir, are both Muslim and refuse to wear clothing with the Proteas' main sponsor, Castle Lager, on the jersey.
The pair have been given special dispensation to wear different uniforms to other members of the South African squad.
BNZ had no issues with his decision. Neither did NZ Rugby. No, it is the public that are the most outraged. And that's a big problem.
It isn't a sporting issue now, it's a religious one.
Even Prime Minister Bill English felt the need to address it.
"It is hard to understand that one guy has to behave differently than the rest," English told Newshub. "If you're in the team, you're in the team. You wear the team jersey."
For those who don't know, English is a Catholic conservative.
Why our Prime Minister needs to get involved defies belief. He shouldn't have this view. They err on the side of ignorance towards other religious beliefs.
Maybe this debacle has a silver lining. It gives us a chance to look at our own societal views. We are, in essence, vilifying a New Zealander for his religious beliefs.
We are lucky to live in this multicultural country but still we feel the need to cut down anyone who is different.
The public is quick to defend the likes of Dan Carter for drunk driving but when Sonny Bill does something to support his religious beliefs we get stuck into him as if he was a pavlova.
Sonny Bill will continue to be the most divisive athlete New Zealand has ever produced regardless of what he does. His jersey this weekend will have Plunket logos on it, most likely some people will have a problem with that.
But maybe, instead of criticising his religious tendencies, we should save the judgement for his play on the field.