Its all about insurance and warranty's
We have the same issues over here.
You can work for a company as a registered plumber and you are covered by their insurance.
You work for yourself, you have to have insurance for the work that you do.
Plenty of registered Plumbers have come unstuck performing work they are covered for working for a Boss, but not covered for working for themselves on the side.
Home owners aren't going to hush up if there is an insurance claim, or warranty claim, on their property regarding work done by a non insured plumber. They will fess up to who did the work and sue him / her for costs of repairs.