In this case there is no smoking gun either way and a natural origin is much harder to trace than lab origin. That lack of absolute certainty, which science is often comfortable working with, leaves room for alternative theories to circulate and attach to those who like the sound of them for various reasons. It is a common human flaw that being aware that an event could hypothetically occur can be mistaken for proof that event actually occurred.
Even if we're 50/50 on the source of the virus, maybe we should still ban or regulate GoF research, and consider banning or regulating some types of wet markets.
As to 50/50 being reason to take large scale regulatory action, would you accept this level of proof to enact any law? That seems a low bar to me.
The point is that we should consider both the cost and the benefit of any regulation, in an expected value sense. Cars do kill people, but they also provide transportation that we've judged is worth that cost. But there's little indication that the WIV's (USA-funded!) risky research has delivered any significant benefit--the predicted coronavirus pandemic has indeed occurred, whatever the cause, and has anything from the WIV's work help us deal with that? On the other hand, even a small chance that their work caused this pandemic is a hugely negative expected value.
Long before the pandemic, there was obscure, academic debate over whether certain types of research with potential pandemic pathogens were worth the risk. Even with the evidence available at that time, I believe the 2014 ban was good, and its 2017 lifting was bad; but that debate now that takes on terrible new significance.