I don't think it's as shaky as you think. ACME is well-known so it would tie back to Warner Bros. and this is a profit-making poster. I think it's quite easy for someone to think it's official and for Warner Bros. to argue it would be losing them business if they make their own posters with their ideas.
"If all mankind minus one, were of one opinion, and only one person were of the contrary opinion, mankind would be no more justified in silencing that one person, than he, if he had the power, would be justified in silencing mankind."
~John Stuart Mill~
I don't think it's as shaky as you think. ACME is well-known so it would tie back to Warner Bros. and this is a profit-making poster. I think it's quite easy for someone to think it's official and for Warner Bros. to argue it would be losing them business if they make their own posters with their ideas.
"If all mankind minus one, were of one opinion, and only one person were of the contrary opinion, mankind would be no more justified in silencing that one person, than he, if he had the power, would be justified in silencing mankind."
~John Stuart Mill~