Could someone please explain this legal question?

There are two different clinics that we can go to – one that’s 45 minutes away and one that’s 20 minutes away. The one farther away is way nicer and they have this service that the other one doesn’t. The service is that you can walk in and say “Hi. I need a strep test. Here’s my money.” and they’ll do it. You can get pretty much any lab work done that way, without the doctor ordering it or even knowing about it. Apparently the closer one chose not to do it because the doctors can be liable for it. How can they be liable for something they have nothing to do with? Could someone please explain the thinking behind suing the doctor, who didn’t have anything to do with it, for something that happened with the test? This makes no sense to me!