The reason why foie gras is not halal is mainly because of the way the geese and ducks are forced fed to enlarge the goose and ducks liver which is seen and is a form of animal cruelty and also contradicts Islamic dietary laws.
In Islamic law they require that animals that are meant to be slaughtered are not subjected to any stress or discomfort before they are slaughtered and that the animals are not deformed, mutilated or diseased.
The force feeding process of the geese and ducks for foie gras causes the geese and ducks liver to become diseased and enlarged and makes it a product of suffering and so it's an unacceptable food under halal standards.
The controversy with foie gras that foie gras production involves cruel treatment, animal suffering, health concerns in humans and also ethical issues.
Creating foie gras involves force feeding of ducks as well as geese to enlarge their livers.
These practices are strongly condemned by animal rights and animal welfare groups as they cite inhumane treatment and cruel conditions for the ducks and geese.
Also people who support production of foie gras often also emphasize it's culinary value as well as cultural significance as a delicacy.
Although the primary concern over foie gras production is the force feeding or gavage process of the ducks and geese, where tubes are actually inserted into the ducks and geese throats in order to pump large amounts of food into he ducks and geese stomachs.
As a result of this force feeding of the ducks and geese it can result in a range of health issues for the ducks and geese, including damage to the liver, impaired mobility as well as increased susceptibility to disease.
Some people also argue that the process causes significant pain as well as distress to the ducks and geese, who are also often confined and also subjected to these types of unnatural feeding practices.
And studies also suggest that human consumption of foie gras can even potentially result in health problems in humans like secondary amyloidosis.
And many people also find that practice of force feeding of the ducks and geese inherently unethical, regardless of whether it can be proven or not to cause any significant harm to the ducks and geese.