And the on-lookers did nothing?
Had it been their fight, they probably would've surrendered…
They were French.
The "onlookers" were unlikely to have been fervent Catholics who were in the building for purposes of worship.
Anyone who has been to Paris will be aware that the many magnificent churches and cathedrals in that city are now little more than tourist attractions. Hence, those present in Notre Dame when this occured would likely have been the usual group of camera-totin', money belt-wearin, sneaker-clad tourists, amongst whom the defense of sacred Catholic sites and teaching would be seen as a relatively trivial issue compared to where lunch was going to be eaten or if the Louvre was going to be open that day.