I am sure this 50% off promotion is only on the fee they charge per transaction but some people here are surprised like they are going to get btc at 50% less price, i think it is not possible for any service to sell btc at half price and regarding fee reduction i think it is a very normal promotion to increase volume on their platform.
Of course, it would be too much of a promotion giving a big discount of 50% for bitcoin buyers. Maybe that can be done with the real pizza (heck even 90% discount can be done with that) but never with bitcoin unless there is a threshold or limit maybe. Yes, it is quite possible that the discount is on fees.