Before they tested the first atomic bomb, they weren't totally sure that it wouldn't destroy the world. They thought it probably wouldn't, but they had worked out the probability that it would. But it didn't (yet ). But yeah, it's a "they didn't know", not an "it could still happen". Do you think if it were the latter, people would be even more sparing with them?

*waits for someone to point out that the above is just an urban legend or somesuch*