The main reasons I don't like to see chameleons sold that young are IMHO that there is a natural mortality rate and inexperienced people will have less of a chance of raising the chameleon properly from that age.

Not all of a clutch will do well as a rule...one or two might not be strong enough and they usually die off before they are 3 or so months old...so if you don't sell them until they are that old there is less chance of a person ending up with one of the weak ones.