Nope Occam doesn't apply to Window Tax as an etymology. Your explanation needs to show that the phrase came into existance during period of that the tax existed, (1696 - 1851) and was used specific reference to the tax during that time, before slowly changing its meaning to its currently understood one.

Occam's razor says that since the phrase "daylight robbery" is a perfectly valid english language construction anyway, there is no reason to look for a complex explanation.

Please note, I'm NOT saying you're wrong, I'm just looking for a cite to prove it.

AndyG