Well there are a few problems with disclosing the actual manufacturer. First off you would be disclosing your source for other vendors to find out, which could be cutting your own throat on some sales. Additionally and most importantly you would be breaking your confidentiality contract with your mfg. Another problem would be "well XYZ corp. says that they've never heard of ABC manufacturing, so someone's a liar, and since XYZ is a major manufacturer you're lying to me trying to make a sale."
The reason that we choose FN contractor instead of Colt contractor is that Colt has done many different modifications over the years and then the question of large pin/small pin, green/grey/black finish, M-4 notches, etc. It saves tons of questions and confusion to go around that issue unless you carry each of the variations that are/were offered (such as fire control parts).
The down side of this, like with anything, are the people who are not completely honest with their sources. These are the companies that give parts distributors bad names. We are attempting to combat this problem by having as many of the major components as possible with our names stamped on them (barrels, detachable carrying handles, hi-rise flattops, our new flattop scope mounts, and soon A-2 and probably flattop uppers as well).
Dr. J - you are correct on the M-4's, Colt is the only company that has a U.S. Military contract for them.