You're assuming nature hasn't found a way to expose such sites, despite all our best attempts at finding geologically stable locations. Obviously we're talking

You're assuming nature hasn't found a way to expose such sites, despite all our best attempts at finding geologically stable locations. Obviously we're talking about a low probability event, but the cost of providing such warnings is virtually zero, so why not do it? (One reason - they might have the opposite effect to that intended. Curiosity killed the ray cat and all).