The thing is that if one of them gets hurt, it's a good chance they'll sue you or put some kind of lien on your property. It's a complicated mess where the no doubt liberal laws allow for shenanigans on the part of the workers.
If someone representing themselves as Pedro's Home Improvement is laying some tile for you and they get shocked because of that plug job and it costs them all sorts of medical expenses, lost of labor and pain and suffering...then it should be really on them. They're their own business after all. Same if they damage your or your neighbor's property. But that's not the way the law is, and it can be different from state to state.
The onus is on the homeowner to see the "business's" worker's compensation and liability insurance, verify with the company that the insurance is up to date and actually has coverage for things. The homeowner must be sure the worker has a license. They may need a city, county and state business licenses as well as a license for the type of work they do. It'll be up to you to make sure they have the permit if needed. In my opinion, NONE of it should be on the homeowner. What do they know about it? The "pro" should know all about it and it should all be on the "pro". But that's not the way the law is.
Funny thing is... if you live in a garishable state, you could have a scammer "get hurt" so he can't work anymore, doctor shop to support his "injuries", win a fat suit against you, have that income garnished from your wages for a long time, and then he can snag some Social Security Disability, some Medicaid for his health, some HUD for housing, and some food stamps for his groceries, and some WIC for his kids groceries.
Here's some sources and there's plenty more on the web.
http://www.silvar.org/index.cfm/article_11.htm
http://www.plaintiffmagazine.com/Sept12/Viadro_Day-laborers-and-homeowner-liability.pdf
http://www.brewerfirm.com/articles/article-OSHArules.html