It is bad for chicks to have too much calcium or too little calcium. With a home-mixed food, you have to avoid both kinds of problems.
There is a big difference between mixing it into their feed (so they are forced to eat it) and providing it free choice. If you mix it into the feed, they are stuck with the specific amount you put in. Providing it free choice in a separate dish gives them some ability to control how much they eat.
High calcium in the feed is known to be bad for young chicks. It causes a number of problems, including unexpected deaths from kidney damage. Feeding "layer" feeds to young chicks would be an example of this (not a good idea.)
Providing a dish of free-choice oyster shell works fine for most adult hens and roosters, and generally appears to be safe for chicks as well. It is common to do that in mixed-age flocks, where the laying hens need more calcium than the roosters and the chicks. If it is safe for chicks in a mixed-age flock, then it would also be safe for chicks in a single-age flock to have access to a dish of oyster shell or eggshell.
