Compliance rate is a very commonly considered variable in evidence based medicine - there is nothing confusing about considering it.
Compliance very much varies between intervention types and cultures. Also not remotely controversial.
Assuming compliance is almost always an error in considering the outcomes of medical interventions.
If we are going to reason about what kinds of coronavirus interventions are effective, we must consider compliance.
Whether something is mandated by law or not is definitely a variable in determining compliance.
If it were not, and as you claim, Swedes have been complying this whole time, why would Sweden have started using force of law recently?
Presumably they think it will increase compliance.