Is it illegal to sell alcohol to someone who is of age, but they have a vertical ID?

So here's the back story. I turned 21 in October. So I can legally drink and buy alcohol. I went to a small store in my area that sells lots of craft beers to do a build your own 6 pack kind of thing for my boyfriend. The clerk running the cashier and the manager both told me they wouldn't sell to me because I had a vertical ID and not a horizontal one and proceeded to tell me that it was against the law......even though I'm 21 years old. And it's still valid. Doesn't expire until the end of this year. I've googled it and couldn't find any "law" that said it was illegal since I'm not under 21. I've gone to other store and they accept my ID no problem. So what the hell? 

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