Here are a few reasons:
Their breeding facilities have been investigated and cited for animal welfare violations dozens of times - not just for ferrets but for dogs, pigs, and other animals.
They sell ferrets to laboratories (e.g. for flu and Covid vaccines since ferrets have similar immune systems to humans).
Their poor breeding practices have likely contributed to higher rates of heart disease, lymphoma, and other chronic illnesses and a steep decline in ferret life expectancy over the last few decades.
They spay and neuter their ferrets far too young, which likely causes hormonal issues and contributes to adrenal disease as they get older.
They (and the pet stores they partner with) sell ferrets to literally anybody who walks in the door, with no knowledge or idea of the care required for an exotic pet. This leads to a lot of preventable ferret deaths and abandoned ferrets.
They continue to breed and sell ferrets despite shelters being overwhelmed (99% of ferrets in the US originally came from Marshall Farms, which means shelters are left to clean up their mess).
They sell ferrets in areas such as Central America that do not have adequate exotic vets to care for them or access to healthy diets.
They manufacture and sell diets and treats that with extremely unhealthy ingredients (grains, starches, sugars) which lead to preventable chronic illnesses and long-term health issues. They also sell some foods with outright dangerous ingredients (peas) which can cause life-threatening emergencies.
They manufacture and sell unnecessary and harmful money-grubbing products, such as shampoos and collars. Shampoos strip ferrets of their natural oils which actually causes them to smell worse. Collars can easily get stuck and choke ferrets. Many of their toys and beds are cheaply made and can cause ferrets to get stuck or ingest pieces of them.