Am I missing something here? From Wikipedia's [Diet of Worms](https://en.wikipedia.org/wiki/Diet_of_Worms) article:
> Prince Frederick III, Elector of Saxony obtained an agreement that if Luther appeared he would be promised safe passage to and from the meeting. This guarantee was essential after the treatment of Jan Hus, who was tried and executed at the Council of Constance in 1415 despite a promise of safe conduct.
So basically, "Last time something like this happened, the guy was guaranteed safe passage and then the promise was broken. Therefore we need to make sure you're guaranteed …