It's strange, my birds eat bananas. Not enthusiastically, but they eat them...
I think not using eggshells from unknown sources is a precaution. There is always a small chance those shells may be contaminated by something you don't want to introduce to your flock.
Much table scraps and "human food" are safe to feed to chickens, but it should always be fed in moderation. Treats of any kind should never make up more than 10% of a chicken's daily food intake. My flock gets cooked eggs, leftover pasta, rice, cooked potatoes, some meat, bread and most other...