Companies pay to be listed on Angie's List and Home Advisor. Those companies rely heavily on feedback from customers. Too much negative feedback or complaints will get them dropped from Angie's List or Home Advisor.
Those online listing companies are basically advertising hubs for these small nameless companies just trying to get business.
When you use these listing companies, they use your location (big brother Google or Facebook or cellphone gps) to link you with a company/companies nearest your location for the work you require.
It's basically a vetting organization as opposed to blindly calling a listing from the yellow pages. They have to do good work to stay listed, and you have a level of leverage when there is some conflict regarding work. They don't want you to report any issues to the listing company.