Well, just tried using Apple Pay at MacDonalds using the same Citibank debit card and it went straight through - no PIN required. Exactly as I was expecting it to work. I did chat with a Citibank rep this morning who told me that it's a security thing and the PIN request shouldn't happen again after the first transaction but I didn't get the feeling she really knew what she was talking about. Guess I need to go back to Walgreens to try it again.
And in response to earlier question, I was only charged once for the transaction at Walgreens that required a PIN entry - although I have seen many reports of folks being charged twice.