Casablanca is a love story between Rick and Ilsa, not a story about a resistance leader trying to escape the Nazis. To say that Laszlo is the protagonist is missing the point of the entire movie. We empathize with Rick because we're following his journey as he pursues Ilsa.
You could design a movie where Laszlo is the protag, and his girlfriend falls in love with the man who has the key to his escape and he has to figure out how to use that to get out of the country... but that's not even remotely close to the movie that exists.
One other point:
In fact, it is what everyone in the story is concerned with - from Strasser to Ugarte to Ilsa to everyone - those letters and who ends up with them are a very important concern to them and the source of problems within the story.
The Protagonist carries the logical argument of the story, the Main Character carries the emotional argument.
I see exactly why Steven thinks that some antagonists are the protagonists of the story, given your definition.
Die Hard. McClane is completely reactive regarding the robbery: he stumbles into it, and by luck isn't captured. He doesn't try to solve it himself - he waits for help, calls for help, kills in self defense.. it's not until late in the movie that he actually actively tries to solve the problem himself.
The contents of the vault and whether or not Hans ends up with them is the problem that everyone in the story concerns themselves with: Hans, McClane, the police and FBI. Hans begins the story by actively showing up and seeking the vault, then spends the story overcoming obstacles to the money.
Obviously Hans isn't the protag, but your definition certainly leaves that possibility open.
The mechanics of the MacGuffin (the contents of a vault, letters of transit) shouldn't define who you label your protag.