Your files are CMYK. The vector files don't have a background and the cmyk32 (8 bits per channel) output device does not have transparency so it makes the background white. If you want to add transparency you should set the ColorSpace to sRGB in the MagickReadSettings:
MagickReadSettings settings = new MagickReadSettings(); settings.ColorSpace = ColorSpace.sRGB; using (MagickImage image = new MagickImage("Porsche.ps", settings)) { image.Write("Porsche.png"); }