You're correct that it doesn't have some wildly divergent meaning from plain English, but it is in fact a legal phrase that has a lot of history and specific meaning behind it:

http://www.law.cornell.edu/wex/but-for_test

http://dictionary.law.com/Default.aspx?selected=113

http://legal-dictionary.thefreedictionary.com/%22But+for%22+...

Are you a lawyer? Have you passed a Bar exam?