The BBC Micro hardware specification includes the clause:
The legend on the keys will be achieved by two-shot moulding
This is a process by which the glyphs are made to run all the way through the body of the key caps, so that they will never wear off. https://deskthority.net/wiki/Double-shot_moulding confirms:
Comptec were one of the world's leading keycap manufacturers, whose keycaps have been found in a large variety of equipment including the Acorn BBC Microcomputer and Wyse terminal keyboards.
The downside is that compared to just printing the glyphs on the top of the key caps, double-shot molding is substantially more expensive. Alan Sugar: the Amstrad story, page 224:
Now a guy at IBM would not think of using a keyboard with printed legends on the key caps. He wants the legends on the key caps engraved in by a double molding process. That means his keyboard is going to be 25 quid, whereas ours will cost three quid. The difference between our keyboard and his is that in n years time the letters on our keyboard may rub off a little bit. But our philosophy is that after three years or so, you can buy another one which will be better and cheaper.
That's enough cost difference to noticeably increase the price of a personal computer. I'm wondering about the process that went into that decision. One possibility that comes to mind is that the BBC wanted to make sure their studio microcomputers looked good in front of the camera. But that's just a handful of machines; if it came to it, replacing them every few months would be orders of magnitude cheaper than using the more expensive keyboard on a million machines.
It seems unlikely that the decision process for that detail of the hardware specification has been documented in a way that survives today, but some insight might be gleaned from looking at the other Acorn machines, the Atom, Electron, Archimedes. If none of them used two-shot molding, that would at least be evidence that this feature of the BBC Micro was a special request from the BBC, not something Acorn would've done of their own volition.