Finding nails and staples all over yard after roof replacement

Lazy workers.

You may want to try calling them back. Show them what you found. Hopefully a few nails are still in the grass waiting to be found. If they're smart - and want a recommendation from you for other jobs - then the boss will come back with one of his better workers to do a magnet search for more. If they won't come back then that doesn't bode well for any future, more serious, repairs.

Magnets. If you have a rake that is metal, you can put magnets on the teeth. I would twine them in place a little, and not expect to be too rough with it. Maybe even use it upside down. Other than that you're looking at a more manual process.

You can rent a magnetic sweeper from an equipment rental yard. I hope the roofers take care of it for you, but if they don't, rent the right tool for the job; don't spend a ton of time MacGuyvering something.

Unfortunately it is not uncommon for workers to be less than careful or less than thorough in clean up. (nor could i say it is common)

Is it "normal" ? There is no way to answer that question.

If they were "trying" to do a clean up then it would seem that there is a company policy in place to do it but maybe it was not executed properly.

Magnet sweeper