Great movie, but why was Brad Pitt given top billing?

Rating: 8/10 (12 January 2013)

I like Inglourious Basterds a lot, but not because Brad Pitt is in it, who as usual over acts and isn't very good. The reason I like it is because of Melanie Laurent and Christoph Waltz
Since both people, Melanie Laurent for absolute definite, have more screen time and hand in superior performances why did Brad Pitt get his face plastered all over the movie and get top billing?
This factor turned me off of the movie when it was marketed and stopped me from seeing it at the cinema, and even after watching the movie and enjoying it as I have done many times, this thing still bugs me. I am surprised that someone as head strong as Tarantino stood for it
Great movie but the Brad Pitt factor prevents me from awarding Inglourious Basterds any more than a 8, 'out of general principle'