can someone please explain the grammer rules involved here?
"Which" is used to introduce non-restrictive phrases. If you remove the non-restrictive phrase from the sentence, it should still make sense and the meaning of the sentence will still be the same, for example, "My favourite animal is the dolphin, which is a mammal." You can remove the "which" phrase and still have a logical sentence, but you can't replace "which" with "that" because you would end up with "My favourite animal is the dolphin that is a mammal" (as opposed to what - the dolphin that isn't a mammal? A reptile, perhaps?)
"That" is used to introduce restrictive phrases. Restrictive phrases are essential to the meaning of the sentence: "The dog that keeps me awake with its barking lives across the street." If you remove "that lives across the street" you fail to identify the dog properly - it could be *any* dog, not necessarily the one that barks all night.
An easier rule? If you could use commas, you should use "which"; if not, you should use "that".
Bokonon obviously meant that Canadians consume a lot of American media and that's why we should know how many states there are. Aside from the fact that "mass amounts" is grammatically incorrect, these two sentences mean different things:
"... you don't need to learn this fact in school, you can pick it up from the mass amounts of American media, which Canadians consume";
"... you don't need to learn this fact in school, you can pick it up from the mass amounts of American media that Canadians consume."
The knowledge can't just be picked up from American media; it needs to be picked up from the American media that we consume.