Using petrol to charge the battery is simply wasting energy. Every time energy is changed from one type to another some is lost due to the inefficiencies of the system. There are 4 states of energy with the Ampera - Chemical (battery), Chemical (petrol), electrical and kinetic (speed). Whenever you convert energy from one type to another some is lost.
When running on battery the conversion is from chemical (battery) to electrical to kinetic (speed).
When running on petrol it is from chemical (petrol) to electrical to kinetic.
To charge the battery there would be several extra conversions in the process... chemical (petrol) to electrical to chemical (battery) back to electrical then to kinetic. This is very inefficient and unnecessary as the Ampera can simply use the petrol directly.
In fact, you are wrong to say that the Ampera doesn't charge the battery it is just that there is no "charge" setting controlled by the driver. The system does keep the battery charged to its minimum state all the time. Recovered energy when regen braking will charge the battery. Mountain mode will keep the battery at about 40% charged or charge it up to 40% if it is below 40% when mountain mode is engaged so it does charge the battery then.
Basically, why not use the petrol directly to create electricity the once instead of converting it, storing it and converting it back?
Simple answer is fuel economy:
If the battery is flat and you drive x miles while charging the battery sufficient to let you drive a further y miles it uses more petrol than simply driving x+y miles on petrol alone. Plus of course it might be that after x miles you reach a mains plug so all that extra charging could be wasted.
What complicates things slightly is that if you're driving along at say 30mph the engine generates more electricity than it actually needs. Running the engine more slowly to generate less electricity is less efficient, so in this situation it will charge the battery a bit and then turn the engine off again and run purely on battery until it has discharged again. Cycling the engine off and on too frequently is again not so efficient so the amount by which the battery charges before the engine cuts out is chosen to maximise economy.
Mountain mode will partially recharge the battery, but again if you need the higher battery level (unlikely in this country) it is more efficient to engane mountain mode before you get to that level instead of burning petrol to recharge.
Very Simply: because if it charged the battery, when you got home the battery would be full of energy that came from petrol (more expensive and not very green) so you could not charge it with mains electricity (cheaper and more green).. So you'd have a 100% petrol car like the prius.