Can an auto mechanic legally really prevent you from leaving without him doing repairs?

I've have several instances of friends visiting automotive repair centers for small problems only to be told by the shop “we found ______ _____ and _____ also wrong with your car. Were not legally allowed to let you drive your car off the lot with out them being repaired. It will be (very large number ) of dollars to do the work”. Is this factual. Sounds sort of like letting the foxes guard the chicken coup.