It's common when corps buy large enough companies that they don't want to kill the original brand. That's why you get hotels like "(something) by Hilton".

Do you mean Red Hat identifies itself using the phrase "Red Hat, an IBM Company"? Because I don't see any use of this on redhat.com (including that website's corporate "about" content) and if any Red Hatters are using this phrasing (I'm a current Red Hat employee) I haven't been aware of it.