I don't say "Happy" myself, but I have personal reasons for that. I don't get bent out of shape when I hear it, and when someone says it to me, I usually reply with something like "thanks! hope you have a great one"
It's not my job to police all that. My job was to, in theory, defend Americans' right to live their lives in peace, with liberty, and with the ability to pursue prosperity and happiness. And if someone wants to say "Happy Memorial Day", then **** it, let em have at it and pass me a bourbon. America