I read here somewhere that they thought that it could cause too rapid of bone growth causing leg problems in young growing chicks. Nothing on adults I have heard of.
Well, my chicks eat medicated chick starter until they are about 5 months, and by that time the others are usually on the grower/finisher, so not really a problem with them. I've not really come across anything related to too much calcium in adults either and that's why it got me curious.