Companies that names themselves something like "the X Company" are following one of the basic rules of English Grammar: always use an article or some other determiner before a singular countable noun. They use the definite article "the" because it's a specific company.

The rule is not so clear that I can give you an easy explanation. All of the sentences (1) "I work for the IBM Corporation" (2) "I work for IBM Corporation" (3) "I work for IBM" are good. In (2), "IBM Corporation" is used as a name so the sentence is grammatically equivalent to "I work for Joe". One thing you cannot say is "I work for the IBM". Consider these sentences: (1) Yesterday I visited Niagara Falls. (2) Yesterday I visited the Niagara Falls. I wouldn't say that (2) is wrong, but (1) is much more common.