Any civilization advanced enough to dig deep enough will quickly understand that the material is dangerous.

Well look, there's only really one civilisation we can look at to see if this is true, and that's our current civilisation. It turns out, though, that this civilisation learned to dig through clay and boulders to any depth a few centuries before it understood what radioactive nuclei do to the human body. It's fair to say a new civilisation would probably learn quickly why all of the people mining near the glowing rocks were dying in pain, but progress in that area would probably be measurable in agonising deaths, which is presumably what people are happy to spend money on these signs to avoid.