I would be displeased. The dog saw this guy talking to you. He shouldn't have snapped at the guy. The guy did not come out of the blue, you weren't shouting at the guy.
Furthermore, if WalMart get's wind of that, they are not going to want people socializing their dogs outside of their store.
So now you know, Instead of getting ramped up at unsolicited advice and allowing your dog to defend himself, be proactive and if necessary, rude. Tell the yayhoo "excuse me, but when I want your advice I will ask for it." or, "The dog is in training, do not approach him."
Your dog may have been keying off of you, but he did not protect when the guy got into your self-space, he protected himself when the guy violated his self-space. Just pointing that out. That is not exactly abnormal. I am not sure if it indicates that the dog is not confident in you to protect him, or if he is picking and choosing who he thinks is ok, not taking direction from you at all.
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