Can't imagine seeing this at other international airports but happy to be corrected.
As been mentioned the use of pyrotechnics airside is fairly commonplace world wide.
For the same reason you're not allowed to smoke airside ... ?
Which is?
I suspect the logic is that if flare pistols and similar were not allowed to be used airside then there's an increased risk of birdstrike. Personally if I was up the front I'd be quite happy the bird scaring was going on and would have happily departed regardless of small fires on/near the runway edge. I certainly wouldn't have refused to depart and then sat there perhaps watching the fire get bigger
