I believe somewhere on the forum is already an answer why is so, also with a quick & dirty hack (google smth like "car charger USB hack" -- dissassemble charger and connect middle wires; note that might be more easy to modify USB CABLE that connects charger and device).

I have 3 useless (at least for N900, N9 and E7) car chargers, from very cheap (5 EUR) to not so cheap (15 EUR) and finally gave up and bought original nokia DC-20 (dual USB charger). Besides beeing big (in height) works perfect!