Seriously, what is different about this issue that they somehow suddenly can’t make the decision that it is illegal taking into consideration evidence other than the load-test?
It is not enough to say, “Aha, that part looks like it’s flexible. Disqualify that car!” It would be impossible to design a part that had zero flexibility relative to the rest of the car, so tolerances are introduced. To ensure fairness and consistency, all the cars have to be held to some objective standard, not the subjective eye of some scrutineer or other.
The objective standards in this case are the deflection tests outlined in Article 3.17 of the Technical Regulations. If a part passes those tests, then it is legal, in practice if not in principle.
The teams design the cars according to the standards laid out in the regulations, so the tolerances allowed in the rules are taken into account when parts are designed.
What should happen is that the FIA takes into account evidence from other sources (pictures, videos etc.) and uses them to design the tests. Therefore anything that is obviously outside “the spirit” of the rules can be clamped down on. In fact, that’s exactly what’s been happening with the Red Bull wings – the FIA have suspected that the wings are flexing and increased the load tests in order to try and catch the team out. But for whatever reason, the Red Bull wings keep passing.
