It's a lawer thing. Handloaders can do whatever they want, but factory ammo must meat SAAMI guidelines as to pressure and dimensions. It's a CYA on thier end.

As for loading your own defensive ammo, I wouldn't do it myself, again as a legal CYA, but that's just a personal opinion. And for the same reason why gun writers make the same suggestion. I also know my moisture control wasn't as good as factory ammo when I reloaded, and had a few failure to fires with my home loads. The only stuff I've ever had fail to fire with factory ammo was WWII or earlier surplus, and the Wolf stuff.