Lay a hose on its side, cut a horizontal slice in it, now sew the sides of that horizontal slice together vertically. You get a considerably widened area in the hose at that point. Voila! Strictureplasty.
Now you see WHY they can't do a strictureplasty if the disease is active at the time in the area. HOWEVER, symptoms aren't necessarily the result of active disease in that area. The symptoms could be the result of a high-grade partial obstruction in the area so that they COULD do a strictureplasty there even if there is active disease in another area.
Some people are like Slinkies... Not really good for anything, but they still bring a smile to your face when you push them down a flight of stairs.