This is just a guess, but because that is such a long bias edge, they use the iron on to insure that there is not stretch when you sew it. It doesn't sound like the iron on strip is very wide - just enough to stabilize the edge. You will sew through it however when you sew the triangle to the square and it won't be removable. If you don't want to leave it in there, you could try a wash away stabilizer. I've never used one so not sure how it would work. Or just starch very heavily and be very careful not to stretch while sewing. Lots of pins will help keep it from stretching, too.