A safety certificate is not a warranty. GP Bikes offering a discount was very kind of them as they likely did nothing wrong. No way a huge business like that is running safety scams for a few extra bucks, they wouldn't risk the license/reputation. That leaves the seller as the POS, as others have said he likely swapped out the good tire for a bald one after getting the safety. It's not worth the fight for a rear tire.
An Ontario safety cert, while valid for 36 days as a piece of paper, is in no way a warranty, or a guarantee of the condition of the bike. It is simply saying that the bike conforms to the extremely loose Ontario safety regulations at the time that it was inspected. Basically it means extremely little.
I'm too lazy to pull up the motorcycle specific tire requirements, but from memory they are the same as car tire requirements, failure if 2/32 (1.6mm) or less of tread, or wear indicators touching pavement. How bald are we talking about here?
GP Bikes is a massive and very successful shop, I have no doubt that they did a proper safety. Safety's are basically a money maker for most shops as it gives them an easy opportunity to recommend and provide repairs. I don't think they would miss a chance to sell a high profit tire. That's not to mention that the tech is potentially jeopardizing his Ontario technician license (essentially his career), and the shop is jeopardizing their Ontario garage license by writing a bad safety. Techs and shops take safeties very seriously.
Bottom line, suck it up, and buy a new tire (pair of tires is always suggested)