They say it "requires Office XYZ" but what they mean is "you need to be able to create and edit Word compatible documents". Most users are going to take the statement "requires Office XYZ" literally because they don't understand the alternatives, and the people saying "requires Office XYZ" are probably even less likely to understand that there are even alternatives available.
This is something that has impressed me about my current college; all of my professors who require their students to submit works digitally have taken the time to point out that their are alternatives to Microsoft and how to get them, namely OpenOffice. Also, all the campus computers have Firefox 3 installed as the default browser, although they have Office 2007 for productivity.
One good suit is worth a thousand resumes.