Because if they don't, they are externalising the true costs of labour to the government, or the community.
Which is fine, by the way, but they cannot then turn around and oppose the cost of taxation needed for gov programs which support people who aren't receiving that living wage. Nor, and worse still, oppose a living wage and then force work people to work such long hours that they cannot sustain a community that can provide the extra support needed to maintain a decent life.
