No one explicitly tried to frame Roger for murder, he was simply a prime suspect as he had opportunity and motivation, so as such the title is incorrect.
That said, "How Roger Rabbit proved his innocence" isn't as snappy a title as "Who framed Roger Rabbit".
The plot (as I remember it) goes like this.
Valiant is contracted by RK Maroon to follow Jessica and investigate rumours of an affair. Valiant takes pictures of Marvin Acme and Jessica playing "Patty Cake", Roger sees the pictures and becomes angry.
It turns out RK Maroon wanted to blackmail Acme (Maroon set up the affair with Jessica), but instead Acme is killed (presumably by Judge Doom).
So a literal reading of the - ahem - facts would lead you to say "RK Maroon framed Roger", but accidentally. In reality RK Maroon set in motion a sequence of events expecting a different outcome. The Bad Guy (possibly unknowingly) hijacked those events and Roger ends up "Framed" instead of simply cuckolded.