The answer is actually that it is done to help to prevent the Sugar Pops(1951 original name) from going stale and from secreting a sticky substance that causes the corn pops to stick together, which you have all experienced it its not a fun solution when all you want is unsticky pops. This problem is caused by the method that the cereal is processed.
According to Doug here is what he found:
The folk at Kellogg's were slightly less than cooperative--perhaps they consider some of these things to be "trade secrets." But we're scientists here. Using a bit of deduction and some good old common sense, it would appear the reasoning is as follows:
(1) Corn Pops and Smacks apparently are the only "puffed grain" cereals Kellogg's makes. (Post also has Super Golden Crisp, which is a cheap Smacks ripoff.) So, they're fundamentally different from the other cereal types, which typically are ground grains.
(2) This difference means that the cereal itself is unflavored (most cereals start as a pasty mix which is then turned into whatever shape cereal is desired), and also a bit prone to drying out, and therefore it must be sugar-glazed.
(3) Sugar glazing is hygroscopic (absorbs water), and if these cereals absorb water vapor, they stick together. You can do this experiment yourself by leaving one of these cereals open in an even slightly humid place.
(4) Evidently, the folks at Kellogg's decided that these cereals got stale too quickly inside the normal bags, suggesting that water vapor can penetrate them. Therefore, they developed the foil-lined bags (which would indeed be impermeable) and solved the problems that way.