All radio frequency energy has electric and magnetic field components. (That's also true of other electromagnetic radiation, such as light!) Transponders are much more effective at creating the magnetic component than the electric component since they work prinicipally by induction.

I think racer1812 had the best observations so far. The very short version is: It isn't a problem until it IS a problem. When it proves problematic, it doesn't hurt to go back to basics, like mounting the transponder as the manufacturer recommends.