Parkour is fine, so long as your not endangering anyone other than yourself.
There is always this argument with public healthcare and how we all have to pay for it (depending on where you live, I guess) but that opens a whole can of worms, like recreational drugs or the whole big brother debate.
(I do mountain biking, so i know this stuff. And let me tell you, going down a set of staires at a 55 degree angle, is NOT the same as a hill )
Yes, Yes, I do mountainbiking too.
But do you also do trial? Do you have a set of 3 inch wide and 5 inch high tires with fat knobs and low air pressure just for the stair riding thing ?
With the proper equipment it can be a lot safer than going down a bone dry single trail at the same angle.
one slight turn of the wheels and they'll land differently to your bike, and whoops, off you come.
Doesnt happen, unless you do REALLY bad. If you get the right speed or are even when going very slow and are in contact with the stairs at all times, you can steer pretty much normally.
Not to mention I can stand still and pretty much turn around, too. Altough I dont really enjoy doing more than a 90° turn standing still on stairs. It just turns into work.
edit; Hey I just remembered this one incident where I didnt descent stairs cautiously so I wouldnt endanger anybody close by, but rather did a full bunny hop at high speed into the flight of stairs, because, well, firstly because it feels hella cool, secondly because I can, and partly because I was listening to fast music.
I checked out the according flight beforehand, it was empty. those 20 seconds later it wasnt. I pulled the bike away from the girl and her dog mid air, landed on my side (thankfully without injuring myself in the process) and never let go of the bike during the whole crash. I never even touched her or her dog...
So yes, I'm pretty confident, at walking speed I will NEVER hurt anybody while going down stairs.