In some places, it IS illegal to sell dyed chicks, or dyed any-other-animal.
I agree that dyeing should not hurt the chick, but it does encourage the "wrong" people to buy the chicks. (People who buy a chick because it is cute, but do not provide proper care.)
As far as practical uses of dye: I wish TSC could get chicks with the sexed females dyed pink, so everyone could tell them apart from the unsexed chicks!
Some states, and some chains of stores, have a minimum of 6.
That causes problems of its own, because 6 is just too many for some small flock situations. Examples: if someone has a flock of 6 total chickens, but 2 died so they want to raise 3 more chicks, or if someone hatched 1-2 chicks in their incubator, and wants to buy 1-2 companions for them.