The 30-gram amount that you find in the legislation is one of the recommendations of the task force on cannabis legalization. This was one of the key design questions that the government asked the task force to consider. After extensive consultations and careful consideration and deliberation, the task force, as it noted in its report, also looked at measures that other jurisdictions such as Colorado and Washington had taken. Its recommendation to the government was that 30 grams was a reasonable amount that individuals could possess for personal purposes.
On September 11th, 2017. See this statement in context.