Two questions
 1. Level of harm from the flash itself-Not a major concern for most
objects-a flash seems bright because of its sudden nature but the amount
of light (compared to the light level in most exhibit areas) is really not
very large. The ban on flash photos in most museums dates from the days
when flash meant bulbs which would explode from time to time throwing
shards that could do serious damage.
2. Protecting  the rights of the institution and  other owner of objects.
That is a valid point the insitution has to deal with.