Because If the concept of cloning is drawn out to the extreme it
is conceivable that particular types of humans can be deliberately
created then cloned to make for instance the perfect soldier,
factory labourer, mathematician etc, etc. Money hungry companies
and power hungry politicians have in the past shown that they are
capable of nearly any access so there are those that are worried
about the implications of even possessing the knowledge to do
it.
Cloning is on the fence with food. Some people want cloned
animals in the food supply, some don't.