For what crime was the young man thrown into prison?

The young man is thrown into prison for falling in love with the princess, the much beloved daughter of the king. The young man is brave and handsome, but lowborn. The princess falls in love with him without the approval—or even knowledge—of her father. In fact, the romance goes on undiscovered, we are told, for many months. When the princess's father, who very much likes to have things his own way, realizes what has happened, he has the young man cast into prison. As the text states:

Never before had such a case occurred; never before had a subject dared to love the daughter of the king.

We see from this action that the king is very possessive of his daughter. He also has a strong sense of social class, which means he is offended that the young man has overstepped his boundaries and dared to love a person so far above him in rank.