I don't know about your setup, but we have a switch on the wall to open and close the garage door - don't need to be in a car at all.
Actually the way you describe it working is a great security measure. Imagine for some reason you have to park outside, I wouldn't want someone being able to gain access to my house by using the homelink to open the garage (alarmed or not).
Presumably if you're leaving the house and want to close the garage or you're approaching the house and need to open it then the car is running, so no problem. If you have a wall switch then you can open the garage before getting in the car and close it before leaving the garage, again no problem.
You are right though that other makes do allow the homelink to work even when the car is off and the keys are not in the ignition.