Please inform your "baristas" that:
Double-cupping is not as effective at keeping the heat from my fingers as the corrugated sleeve is (as my fingers will testify after the half mile walk to work). If it was, you wouldn't have gone to the effort of having the corrugated sleeves created for this very purpose.
It's not particularly good form to tell your customers they're wrong, even if they are. Which I'm not (this time).
Tell them thanks for helping to start my day with an argument.