The "Do Not Reuse" thing is a CYA, legalese disclaimer. You can reuse them once, maybe twice, but, beyond that, the 'lock' feature doesn't work as well, and it's possible they could loosen, or come off entirely. Manufacturers would just as soon their customers stayed alive, to continue buying parts/cars/trucks.... so, they would much prefer you replace the 5 dollar nut, just to be safe.