Was this their mistake? Should they have caught this or am I expecting/trusting too much?

The check engine light came on as well as the oil light a few weeks ago. My husband checked the oil it was low, so he filled it and then we took it to my mechanic the next day. They replaced the oil plug and sent us on our way. Several weeks later, with no engine or oil light on at ALL, my engine start to make this knocking sound. I pull over immediately and check the oil. Almost bone dry. I'm shocked - I thought this was fixed?? I get some oil nearby so I can drive to the mechanic, but decide it is best to stop halfway there and call the mechanic for help. They are not helpful AT ALL. Deny they did anything wrong and tell me to pay to have it brought to them so they can fix what is wrong. They say if it is a problem with the plug then they will reimburse me for the tow and fix the plug but that is all they will do. Is that fair? Who is responsible for the tow? Who is responsible for this problem? Should they have been able to determine if the plug really was the problem in the first place and not something else? Something that they missed the first time...making them responsible for the consequential problems. Other helpful info might be that it is not leaking oil...no spots on driveway at home were noticed over the several weeks after repair (or before for that matter) or where I parked it after knocking sound was noticed.