It's the whole descriptive vs. prescriptive thing, and MW explicitly adopts a descriptive standard (they say so in their preface). A good introduction to this larger question is David Foster Wallace's (excellent) essay in review of Bryan Garner's (fantastic) usage guide (http://harpers.org/wp-content/uploads/HarpersMagazine-2001-0...).
If you are interested in usage, and are not familiar with Garner's book, I recommend it. (http://www.amazon.com/Garners-Modern-American-Usage-Garner/d...) It is superior to MW for most writers' purposes. (For students of linguistics, MW might win out.)
Garner, incidentally, doesn't much like "comprised of" -- http://www.lawprose.org/blog/?p=2385 (“invariably inferior”).