Syntax - you take a rather dim view of high end cable makers' integrity it seems ;-) not that one shouldn't be cautious, of course -- however, due diligence can clear up a lot of assumptions.
Actually, Purist uses a paste called Ferox, made with powdered iron oxide, to fill the jackets surrounding their gold/silver alloy (not plated!) conductors. Why did you assume it was 'cheap foam'? ;-) Ferox is quite heavy, but flexible. Purist has in the past used fluid-filled jackets as well, and now have sort of combined the two in a material called Contego. In any case this design very much contributes to a really clean music signal. If you ask people who own Purist products, the first thing they'll tell you is how quiet they are.
I'm not sure what materials Silver Audio uses but its a similar design, i.e. mechanical isolation + electrical shielding combined into a homogeneous substance.