Poor Marie.
I don't blame the Prince at all though. Marie herself says that if she were anyone but Princess Morina, she would be elated to receive that reward. In fact, she wishes she wasn't Morina, because she wants to be happy for this development. I think she's grown to respect the Prince, and if she didn't have to be afraid for her life every day, she would enjoy her life at the palace much more.
Unfortunately for her (or not, since she will no doubt get a happy ending), everything is very transparently happening for the sake of plot. Writing a good story requires creating obstacles that prevent the protagonist from reaching their goals. And in this case, the author has made it so that Marie can't refuse the title or even explain to the Prince that she wants to live a peaceful, quiet life in obscurity.