Why did Duke Karl Eugen forbid Friedrich Schiller from writing plays?

Duke Karl Eugen of Württemberg did not forbid Friedrich Schiller from writing plays because of the content of his work, but rather as a disciplinary measure for unauthorized travel.

After the successful premiere of his first play, Die Räuber, at the National Theatre in Mannheim, Schiller traveled there to attend the performance without obtaining the Duke's permission. Upon learning of this unauthorized visit, the Duke sentenced the poet to a fortnight of detention and issued the prohibition against his further playwriting.1

Schiller, who had endured years of regimentation under the Duke's supervision at the Karlsschule, ultimately chose to flee Stuttgart to escape this restrictive environment and continue his career as a dramatist.1