There are two main reasons that scratch can be bad for chickens.
One is that the chickens get fat from overeating. (As in, they eat more total food than they otherwise would eat.)
The other is that they fill up on scratch, and do not eat enough of their other food. That leaves them lacking some of the nutrients they need.
Scratch can be part of a healthy chicken diet if it is planned properly. For example, about a century ago, it was common to feed some scratch along with a mash that was specifically made to go with that scratch. When the chickens ate the correct amount of each one, they received a balanced diet. Modern chicken feeds are more likely to have everything ground up together, with the correct amount of grain mixed into everything else. That is very convenient for people raising chickens commercially, and for backyard keepers that do not want to think about balancing several different kinds of foods, but it can make things harder for people that want to incorporate other foods into the diet of their chickens.
