Sir I am simply attempting to answer your question with a valid answer. They can ID anyone that they please. Whether they appear of age or not. It is called verify age. It may well have been also that the employee was new and wanted to be a little over on the id's. If she showed an attitude or you feel this was wrong then speak to the store manager tomorrow on this. He can offer an explanation of their store policy for such checking of ID's. You may also contact your state ABC.
http://abc.state.ny.us/JSP/content/faq.jsp
Forensics/CSI
Sworn Expert Witness in Criminal Law. Sworn Expert Courtroom Testimony.