Why should I have to prove that nothing was standing there before? I have never signed a disclaimer on any construction project stating that nothing is allowed to be placed in front of the windows. I also can't imagine that this is universally valid?
Because almost all glass/pane/window manufacturers explicitly point this out. Stress cracks are explicitly – and unfortunately universally – NOT covered by the warranty. You can feel free to inquire with manufacturers and craftsmen about this. If something is done, it is as a goodwill gesture. You only have a claim if you can prove incorrect installation. It’s unfortunate, but it cannot be changed.
I will give you a conclusion from the Federal Association of Experts in the construction glass sector: CONCLUSION: "Thermal glass breaks," "mechanical glass breaks," or combinations of both have various causes. As of today, there is no guarantee to completely exclude these risks. In the event of glass breakage in an installed condition after the final inspection, neither the glass supplier nor the window manufacturer is responsible. However, thorough planning can recognize and avoid most problems in advance.