Their existence is almost always protected by law in e.g. the US, but de facto and de jure are not the same thing, and as with many relationships involving asym

Their existence is almost always protected by law in e.g. the US, but de facto and de jure are not the same thing, and as with many relationships involving asymmetric power, the risk/reward and burden of proof may be too high to justify the actual legal case about illegal actions versus quietly settling things for more/less than you would have originally earned, but with much reduced legal fees.