We don't know details
1. There might have been a skirmish before.
2. Emperor has right to interpret Imperial Law to whatever fuck he wants it to be. Like "healing my immolation in a city is basically the same as striking me in the city, hence rule # 234 applied".
Caecilius is playing a very thin edge bastardly role, and the player is VERY knowledgeable with imperial law nuances, that's for sure.