Actually, there’s a good reason. Steven Pinker, the famous
MIT linguistics professor, spends a few pages in one of his books
explaining why “Maple Leafs” is correct, and more natural, than “Maple
Leaves” in this context. It’s the same reason that we say that a
baseball player “flied” out when he hits a pop fly, and not that he
“flew” out, even though the normal past tense of “fly” is “flew”.
The idea is this: If an irregular word is removed enough from its
original meaning, often by going through different parts of speech, or
by becoming part of a larger unit, it tends to become regular.
In this context, a Maple Leaf is not a leaf at all; it’s a type of
person. So it loses its connection with the irregular word “leaf”.
Another way to put it is that it’s the whole phrase “Maple Leaf” which
carries the meaning; it is no longer the noun “leaf” modified by the
adjective “maple”. And noun phrases cannot be irregular.
In the case of “flied” out, that comes from the noun phrase “a fly
ball”. When the verb “to fly” is made into part of a noun, and then
reconverted into a verb, it becomes regular.
Another example Pinker gives: If I were to say, “That person is a real Mickey Mouse”, meaning that I think he’s stupid, how would I make the sentence plural?
If I said, “Those people are real Mickey Mice”, it sounds very strange. It’s more natural to say, “Those people are real Mickey Mouses”.
This isn’t because of some rule of grammar. It’s (according to Pinker) because of the way the brain handles language.