It was a mistake for Duolingo to introduce Portuguese sentences that required learners to understand the difference between "used to " & "to be used to" since Duolingo is itself confused.
I think that "eu estou acostumado a beber..." is more appropriate
Why does it not accept "beber"?
It accepted "beber" for me (April 3, 2014), so maybe they got enough reports and changed it. :)