A PROFESSIONAL martial arts business owner has responsibilities - financial and legal and otherwise - that the amateur martial arts enthusiast does not.
I'm a professional sifu. As far as I'm concerned, it is not ethically or morally on another instructor if his students come into my school and challenge me on HIS behalf. A sifu does not "make" a student walk into another person's school and cause trouble.
The law would recognize that those students acted on their own accord. I have to check my texts, but my understanding is that there would be no legal recourse taken against a sifu whose students acted improperly in another's school.
My lawyers have told me that if I'm physically attacked, I have a right to defend myself. How much force and what tactics I use to defend myself will be taken into account by the judge and jury during trial proceedings. The law does not look kindly on those who injure people above and beyond the call of the situation. I'd also go in very PREPARED, legally speaking.
Concerning the sifu-calling, I would call a sifu about a student of his that attacked me in his name. I would do it out of professional etiquette, to notify this teacher that he's got a troublemaker on his hands that would be spoiling his business's reputation. If more trouble ensues because of that, the Better Business Bureau would be the first to know.
Now if somebody comes into my school and politely asks to try me in sparring, I would GLADLY oblige if we laid out some ground rules beforehand. I as a martial artist and school owner accept that as the risk of doing business and would likely even sign a waiver annotating the responsibilities of entering such a situation. That's to cover my opponent's butt as much as my own.
A good kung fu man knows the parameters of his fight before even THINKING about fighting.