The reason is that the Flamethrower graphic is an overlay in the iscript, not an actual graphic.
If you want to have the SCV use the flamethower, you have to add this line to its GndAttkInit animation with IceCC.
CODE
imgol08 421 0 0 # FlameThrower (thingy\flamer.grp)
EDIT: DoA snuck that reply in when I was decompiling the iscript!