I don't care that they lock it up. I only care that they want you to pay for it there, at that counter, or walk you to the register with it. Annoying! That means ammo has to be my last stop, I want to get it first.
Once time, at one walmart, the ammo counter guy was a personable, older gentleman. He told me, the only reason they do that, is so someone doesn't change their mind about purchasing it, and set it down in some random part of the store where it doesn't belong. I had marched right up to him and said, "Two 100 round packs of WWB in .40S&W, please." I must have looked determined, as he said I could just put them in my cart because I looked like someone who was sure exactly what he wanted...
----FWIW, I've never waited more than a minute or two for someone to open the ammo case, but then again, I'm the sort of guy that, if there's no one behind the desk, I'll go out and find someone.