Main account holder has to be there. Same deal with Best Buy (I asked). The Authorized User thing is only for corporate AT&T stores.
You are right.
"Retail Authorized Users and Apple Stores - If visiting an Apple store, the primary account holder must be present."
- from AT&T's support page: http://www.wireless.att.com/answer-center/main.jsp?t=solutionTab&solutionId=KB90110