There has to be a way around that, its completely unreasonable.
Who is NOT going to have their N900 on charge while in the car?

Its totally insane to expect people to drain their batteries while near an outlet. Especially as iPod FM transmitters come with car adapters, although I think they factor in the cable as you get no reception at all without it plugged in.

But that just proves the point, surely you just factor in the likely signal boost from the cable and drop the output strength accordingly while charging. There is always the possibility of interfering with other people, it all depends how well shielded your car is to begin with.