Slitherine

why do you get a - for shooting at a bg but don't 1 for impact or melee

Cover hinders shooting effectiveness, but the ground condition effects close combat. Consider, a BG on a bare steep hill is not in cover, so shooting at it is just as effective as if it were in open ground. A BG in a plantation or a forest would be in cover, but would be in rough or difficult going respectively for close combat.

so what would happen to the POA if you where fighting a bg who was behind a wall?

Are we talking FF or a terrain feature?
In FoGR you are probably looking at a linear obstacle, but I don't think (no rule book with me) that exists in FoG:AM, in FoGR it is treated like an FF.
If you are doing a refight/scenario, then you could work out your own modifiers as appropriate.

FF represented by a wall gives a POA modifier against shooting as well as against enemies in close combat. A wall that is used to denote an "enclosed field" terrain feature merely is part of the terrain feature and is treated more abstractly. Troops entirely in an enclosed field are in cover for shooting purposes, and close combat in an enclosed field is in rough terrain, which disorders HF and most mounted, severely disordering Knights and Chariots.