ACtually AC could have accepted the tickets. The 'no sticker' policy only referred to their ticketing stock and there is no way that AZ would have known that.

AZ could have also just reissued the ticket properly so that it reflects as a paper ticket instead of a sticker. You could have explained to AZ that AC says that the ticket needs to have a printed date and that as you are flying in Business Class, you want the tickets reissued properly into tickets that didn't have stickers on them.

Onus is on AZ for refusing to help you... and it was their ticket , for G*d's sake! Why wouldn't they reissue the tickets?

I think you meant to say that although the flight was on AC, your ticket has a UA flight number right?