I looked over some of the bolts I have on hand.
The Horton Lightning strikes have a blunt aluminum end with no hole in the centre, so they're clearly a special device.
The Excalibur bolts I just bought have a plastic device in the end that looks like a tip insert and is threaded - they may be a plastic tip insert. They sell a device ("T" handle) for threading into the base of the bolt for extracting them from targets, so the threading is desirable.
All of my other bolts - ones that I bought from archery shops - have regular aluminum tip inserts in their bases.
So, it seems to me, that you can either order special inserts, and use them, or use tip inserts.
One of the advantages of having a threaded insert in the base is when it comes to re-fletching your bolts. Most fletching tools require that a nock be used to hold the arrow in place while the job is being done. Since bolts (except for the Bear) don't have nocks, these tools can't easily be used for crossbow bolts. With a threaded insert, you can make up an adaptor that allows you to screw a nock into the base of the bolt while you are re-fletching the bolt.