Why does my parking brake disengage when I shift into drive?

You probably shortened the life of the rear brake shoes and drums a bit, and may have overloaded the engine and trans enough to just barely affect their service life, but mostly what you did was burn extra fuel. Check the parking brake on a pretty good grade, with car in neutral, to see if it still holds. Might have stretched the cable -- easy adjustment.

The parking break is a manual brake operated by a cable to the rear brakes, it may or may not disengage when you put the shifter into drive depending on what type of features your car has..

Since the car was accelerating just fine then Most likely you didnt have the parking break completely on enough to stop the vehicle or you have the safety feature, in any case you most likely didnt do any damage to anything.

Your parking brake did not disengage. The parking brake only applies the brakes to the rear axle. Car engines are powerful enough to overcome the parking brake. I don't know of any cars that are not equipped with a parking brake - it also functions as an emergency brake if your main brakes fail. Yep, it is definitely something you should ALWAYS check. The brake light comes on for one of two reasons: 1) parking brake applied, 2) fault with the main braking system

its a safety feature so you do not drive with the parking brake on. Most cars engines are powerful enough to overcome a simple parking brake. The reason is the parking brake is to hold your car mainly on a hill or something. And there is no need to have it on and drive like that. Its working fine Im assuming so do not worry about it if it aint broke don't fix it.