It’s called policy.
If the kit they are issued with is deemed “fit for purpose” then if they want something else it’s buy your own. That’s why Silverman’s do a roaring trade.
However, this policy is routinely stretched. Kit is not always fit for purpose. In fact, basic kit is not always supplied in the first place. Not just what you wear but also what you carry, use and consume.
The aircraft world is actually quite well served in this respect. You may buy your own pair of boots, but when was the last time you had to buy and fit an antenna because your radio didn’t work without it and none was supplied? A no-brainer you would think, but this is a well-known scenario faced by vulnerable troops in theatre. Thank God for some companies I could mention who effectively “sponsor” units by supplying such ancillaries gratis. The issue of Kevlar you mention was, of course, key to the tragic loss of Sgt Roberts.
It’s also called abrogation of Duty of Care.