Trust me, value for money didn't seem to be the primary concern of whoever designed these keyboards.
Value for money always is the primary concern of whoever designs electronics. It's just cheaper to use a material with a vastly different dielectric constant than to beef up measurement electronics to be able to accurately measure tiny fluctuations in capacitance when using standard plastic.
This doesn't compromise functionality at all, but you just don't need lab-grade equipment in a keyboard. There could be many ways of alternative keypress detection, but it'll just become horribly expensive when you need accurate equipment to measure it.
Also, burning plastic in a bonfire might be fun, but it won't tell you anything about the plastic. Best test is to heat it gradually and look for the temperature it starts to deform, then to burn it on its own, using just a small ignition source (quite hard with flame retardants), note the temperature at which that happens and observe burning characteristics (smoke/soot), smell etc.
PVC should be quite easy to tell from the smell.
-huha
