I don't want to say it's a scam, but they manipulate the prices during these sales so you're basically paying the same.
Depending on tHe retail store you are dealing with, that's possible.
The way it works north of the border looks like this:
Me (Distributor) -- I charge the tire store for 4 tires at their normal cost, then immediately credit them back for a single. This price does not change during the promo. (At least not with my company, we actually lowered it a bit because of a competitor.)
Retail Store -- They set their own price markup from what they pay the Distributor, then charge out 3 tires are their normal markup, and charge a 4th at 0 dollars.
The retail stores are instructed to sell the tires "at a reasonable level" and are required to keep copies of all of their paperwork.
These loose guidelines are open to abuse.
The tire shops could charge full retail price (no one does this) or they could sell the tires to their friends at their own cost.
If Pro comp sees some unusual buying patterns, they will request the backup paperwork and if there is a problem, they re-nig on the deal and charge them back for all 4 tires.