When redstone dust is placed next to the side of a comparator, the dust connects and the comparator uses its signal as input. However if the signal is coming from a block with redstone dust power or a repeater pointed into it, the comparator does not take the input, and just ignores it. I would expect the comparator to take the side powered block the same as it takes a signal from dust or a repeater directly into it. The images show the comparator in subtract mode to better show the problem, as a signal of 15 will turn the output off. The images show a block that is powered does not work as the side input to the comparator.