“Second, when you care concealed, don’t flash the gun.”
No sign he did. If he was carrying in a fanny pack - as it seems folks say he was - then the clerk who saw him may have asked to see inside if she was concerned about possible shoplifting. The options are to refuse and be stopped for suspected shoplifting, or comply and thus ‘flash’ the concealed weapon inside. Or you then tell the clerk you are carrying concealed, and now she knows anyways.
In a hot climate, it is very difficult to perfectly conceal a weapon at all time. Go to a forum that discusses concealed carry, and ask if anyone has ever accidentally revealed a weapon. I think most will tell you that if you carry long enough, you WILL ‘print’ sometime.
In 2009, 48 cops were killed in the line of duty. There were roughly 750,000 cops employed - the estimates vary. Stopping a guy coming out of Costco isn’t exactly a high-risk proposition...it isn’t a drug bust, or home entry, or dealing with a bunch of drunks at a bar at 1:30 AM.
They had the option of doing it in a less crowded area, and didn’t need to be all prepped to shoot. They might even have started by asking a question - what a concept!