this is only the case with programming credits (like the 50% off programming option currently on new rentals)...since the programming is $100, you get it for $50, so you are only paying tax on the $50, which would end up at $57 in ontario (14%).
in the case of the half price purchase option, if you buy a $599.99 system, you will get a credit applied to your bill of $300...if your bill is $114 with taxes, you will show a balance of $186 as a credit...credits are applied to your account just like a payment is...the "net" pricing shown in ads is never really correct, as it doesn't take into account the taxes...the $599.99 price, would mean you would pay $683.99 and get $300 applied to your bill as a "payment" in the form of a bill credit (same as paying $300 cash on your bill)...your net price would be $383.99 including taxes, instead of $299.99 plus taxes which would be $341.99...perhaps it's easier with smaller credits...if your bill is $114, and you purchased the 4120 system, so you would get a $50 credit, when you got your bill in, your bill would be $114-$50=$64.
it is the same way cell phone credits were processed before they moved to have them done in store...you use to buy your phone for $250 plus tax, and if you signed a 2 year contract, you received a credit on your bill of $150, but "lost" out on the tax difference on the $150.