Premier Kathleen Wynne wrote to the organization last week, saying she had struggled with striking the right balance between public safety and religious accommodation.
"After careful deliberation, we have determined that we will not grant this type of exemption as it would pose a road safety risk," she wrote in the Aug. 14 letter.
"Ultimately, the safety of Ontarians is my utmost priority, and I cannot justify setting that concern aside on this issue."
The mandatory helmet law is based on extensive research that shows the high risk of injury and death for motorcyclists who ride without a helmet, she added. Mortality rates have gone down 30 per cent and head injury rates down 75 per cent in jurisdictions with such laws.
Courts have also found that Ontario's law doesn't infringe on the Charter of Rights and Freedoms or the Ontario Human Rights Code, she said.
The association has been a strong advocate for an exemption and presented "compelling arguments," Wynne wrote.
"However, the Ontario government has carefully monitored, and considered, the soundness of accommodating your position, drawing on relevant academic research, key legal decisions, and consultations with caucus and the community."
The organization said it felt let down by the Liberal government, which had promised to bring in legislation that would provide the exemption.
"In all our discussions and meetings and consultations, we were given the understanding, assurances, commitment that we will be moving forward on this," said Manohar Singh Bal, secretary of the association.
Members of all three parties as well as other high-profile Canadian politicians, such as former premier Bob Rae, all support the exemption, he said. Former transportation minister Glen Murray promised to introduce a bill sometime in March or April this year.
"The premier has reversed her position, I will say, or has not followed through on the understandings and commitments that she gave," Bal said.
British Columbia, Manitoba and the United Kingdom have all enacted legislation that allows turban-wearing Sikhs the right to ride a motorcycle, he said.
Wearing a turban is part of their religion and dress code, he said. They can't go out in public without wearing one.
"It's part of their very being as a person," Bal said.
Ontario Provincial Police allowed its uniformed officers to wear turbans long ago, but the province won't grant Sikhs equality when it comes to the helmet law, he said.
"It's mindboggling how they pick and choose where they want to accommodate Sikhs and where they don't want to accommodate Sikhs," he said.
"It's just like we're at the mercy of the majority that they will pick and choose which rights we are entitled to and which rights we are not entitled to, and this is a classic example of that."
Almost a year ago, Wynne spoke out against Quebec's controversial "values charter" which aimed to impose restrictions on religious clothing and symbols on public servants.
Her government introduced a symbolic motion promising to oppose any bills that would restrict people's freedom of expression and religion in public places, which was passed unanimously in the legislature.
NDP MPP Jagmeet Singh, who introduced a private member's bill to support the helmet law exemption, said he's also disappointed in the Liberals and will continue his efforts to protect all articles of faith.
"While the Wynne Liberals are happy to pay lip service to civil rights, when the rubber meets the road, this so-called activist premier is quick to deny the Sikh community rights recognized elsewhere," he said in a statement.
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