Answer:
1. They keep casts to preserve the real fossils.
2. Maybe someone would steal a fossil and sell it on the dark web?
3. They keep casts to keep the real fossils in prime condition, by that I mean like undamaged.
The answer is confirmation bias. This is the tendency to look for evidence that will confirm ones hypothesis and ignoring anything else that does not lead to the hypothesis. In this situation confirmation bias is evident due to the fact that you look for anything that confirms your hypothesis and ignore anything else that does not point to drug abuse.
