They aren't cheap, they are small sheets of polyurethane foam being sold for $50.
Here is a mattress sized piece being sold for $45 shipped.
Here is a good sized chunk for $15. Or you could go to any foam packing place and get a subdude sized sample for free. As for the physics, how does a 100 to 300 gram part oscillating at say, 15 mm 35 times a second in a 50 lbs braced, damped cabinet going to vibrate loose stuff on the walls? A sneaky mechanical chain of contact from the cabinet to the floor to the wall to the shelf to loose object x? Or, directly through vibrating air, like what a subwoofer is made to do? Furthermore, physics is a science, and science relies on repeatable quantifiable observations for validation, which is not something subwoofer isolation pads have in their favor.
You would think in such a objectively weighted audio site there would be just a slight bit of skepticism about over-priced gimmicks with zero evidence behind it, but I guess isolation pads get a pass because they sort of seem 'sciencey'.