Why are AK muzzle breaks that are sold today threaded (and not pin-on like they used to be)?

During the time the "Assault Weapons" Ban was in place (94-2004) all muzzle breaks and flash suppressors were required to be pinned and welded into place. Once the ban expired threaded barrels became popular because they allowed you to select your own brake/suppressor rather than being stuck with the one that came with the gun.

There are both screw-on and pin-on types available. You need to look around more....I found these in two seconds by Yahoo searching with "AK pin-on muzzle brake"

Yes, expect it to be "gunsmithing required". Look, these guns were made over the course of many years in dozens of different factories with the toolings all different. You need a muzzle brake to be tight as ****. So expect that you will need to fit and file to get a snug application. A loose one would be very bad....

You can find both for sale on several websites.