Since I ate your duck for dinner, the beer is yours. It's the least I could do.
Syntactically, "it's your beer" and "the beer is yours" are quite different. The intended meaning might be the same, depending on context.
Because that's not what the sentence said. Suppose you're at a picnic and someone brings a tray with a beer and a coke. Which one is for you? They say "The bee o is yours", not "It's your beer." As has been mentioned, the phrases could mean the same, but might not. Translate it the way it is written, with beer as the subject.