Yeah it’s fine. I actually put two staples in, with a little gap between for the air and steam to come out. In a pinch you can leave the staples out but usually the pressure of the popcorn unrolls the bag and the popcorn comes out.

I’m actually an antenna engineer (radio frequency/microwave engineer) and I’m embarrassed to say I don’t fully understand it. I’ve seen actual metal racks that come with microwave ovens.

Theoretically what would happen is currents would flow on the surface of the metal to what we call the skin depth. Since the power level is high, those currents would generate heat. If it’s enough heat the metal could melt or even pop. When this happens it can cause a burn mark and mess up the finish of the interior.

The other effect would be that the microwaves will reflect off of the staple. Reflected waves shouldn’t really do anything except cause the wave to bounce around in the microwave, which they already do. The inside of the oven is metal after all.

I think they say not to do it because thin metals can melt or burn. Thick metals might reflect the waves away from where you want them so your food doesn’t cook. Otherwise I don’t see the harm but maybe I’m missing something. I design antennas, not microwave ovens, but from experience I haven’t ever had a problem with the staples.