So---hypothetically---if free codes weren't posted would this contest be as hard to win?
Probably not nearly as hard. I'm sure their selection algorithm adjusts based on the number of entries. Without the codes the number of entries would probably fall significantly. Pumpkin Fright Rider wasn't nearly as hard until the end when lots of people were getting their free codes. But, they made the mistake (intentional or not) of having to make the codes re-usable by printing them on the outside of the bag and putting the same code on many bags. The worst possible thing would be for someone to buy a bag and be told their code was already used when they got home - it would be a PR nightmare, not something you want on a promotion.
It probably has an expected internal win rate (to make sure they don't run out early, you don't want a two month contest to run out of prizes in two days), and when entries don't pass a certain threshold it improves the chances until it is practically a guaranteed win. More entries and it lowers the chances. I would think chances would get better later on once all the "gotta win four or more" people have their fill (and others get frustrated and quit trying). It was part of what I was saying, if people won one then stepped out the ratio likely improves for everyone still playing. They almost certainly are not giving away more than they want to give per day, and probably have it set up to make them available in the one per fifteen minute period they state in the rules.
Of course all of this is speculation, they aren't going to make this stuff public, it would make winning easier to predict. In the end though, they want to give them all away, and also have people win as often as possible - they want these given away, it is a promotion after all - they have already paid for all of them.