Carded for Alcohol-Removed Wine??

So I didn't even think to bring my ID and was getting alcohol-removed wine for a baby shower. Cute right? Well I couldn't get it without an ID and I'll admit I was a little pissy about it. Now I'm reading that the reason is they can't guarentee the alcohol level and everyone says it's probably safe for pregnancy but there's a tiny chance that it's not. As an over-cautious paranoid person I think I'll opt for sparkling grape juice but I'm wondering does anyone know why they can't measure the alcohol content accurarely?