In many industries, salespeople earn a living thanks to commissions, which can often get characterized as kickbacks. However, sometimes commissions or kickbacks aren't necessarily a good way to sell. They may not even be a legal way to sell, particularly in heavily regulated sectors.
In short, a kickback is illegal when there is a law prohibiting it, or similar conduct. These will often exist in the context of consumer protection laws, as well as government funding. And if you aren't sure whether your commissioned sales team is being paid legit commissions or illegal kickbacks, you'll want to read on.