Why do metallic fibers keep poking through my new shop slab?

The short version: your contractor is right that it's normal, but he's wrong if he didn't tell you beforehand. Fiber pop, wich is what those little whiskers are called, is a known thing with steel fiber slabs. The fibers want to lie flat while the concrete is plastic, and machine troweling ussually tucks them under the cream pretty well. Hand finishing along edges and corners is where they poke up, because a guy on his knees working a steel edger tends to drag fibers to the surface instead of pressing them down. So the pattern you're describing, clean field and fuzzy edges, is exactly what I'd expect.

Had the same thing on a shed slab I did with steel fibers a few years back. I just ran a grinding stone over the whiskers once the slab had cured a few weeks and it knocked them right down. The orange halos I got are still there though, they faded some but never fully went away, so if you care about the floor looking clean I'd grind those spots sooner rather than later. Sealing the slab once you've ground it flat helps too, keeps any remaining fiber ends from rusting. For what it's worth my slab has held up fine otherwise, no cracks or anything, so the fibers themselves are doing thier job.

On the rust worry, my understanding is that once the surface fibers are ground flush they stop being a problem in a dry slab, because steel only keeps staining when it can pull moisture from somewhere. The halos you already have came from rain sitting on exposed fiber, which basically acts like a little wick. One thing I'd ask the contractor is whether the fibers went in at the batch plant or were tossed into the truck onsite, since onsite additions are where you hear about clumping and poor distribution, and I'd want to know which I paid for.

Update: went with the grinder suggestion and it worked better than I expected. Waited a few weeks like the second poster said, ran a grinding stone over the edges, and the whiskers are gone. Halos by the door faded a bit but they're still faintly there, which matches what people said. Rest of the floor stays dry so I'm not expecting more rust. Live and learn on the heads-up part.