The idea is that some people with disabilities are going to be unable to find work where they can produce value higher the federal or state minimum wage. It’s possible such work exists, but maybe not where they live or maybe not in something they’re trained in. A job is better than no job, for more reasons than just money. Work gives people the feeling of earning their way, like they are valuable to others in their own right.

Whether such a policy is actually fair depends on specific empirical facts, not some a priori judgment. You’d need to know the economics of labor by people with different disabilities, the ability of companies to accommodate those disability, and so forth.