Sorry to be picky, but .com does derive from commercial, as .edu is from educational, .gov from government. Even as .com, .edu and .org are top-level domains, they are at the same level as .it, .uk, .br and .us for that matter.
The country-specific top-level domains came along only when the internet became widespread accross the globe, but at that point the US had already many domains registered without country references. It is common therefore for US sites to use these domains, but it's not a rule as many British companies do use .com as well.
The .COM extension in MS-DOS has no relation to the .com top-level domain.