Of course, thousands (or millions) of years ago petroleum used to be biomass, but at some point the effects of the usage of things change and therefore it makes sense to differentiate.

If you ask me to build you a terrace out of basalt plates, you probably wouldn't be too pleased if I conduct a stream of lava through your yard although basalt has been lava at some point.

Fossil fuels are segregated from biomass to avoid confusion of material types. Biomass is much more cellulosic and has not undergone heat and high pressure transformation like petro materials.