>>Reality check... why do they have to check in the 1st place ?

Different countries have different drug issues. Pot is widespread in NZ, as is P, or methamphetamine. Locally there's good evidence that both of these alter behavior wrt safety, along with alcohol obviously. Which is why NZ Health and Safety authorities identify pre-employment drug testing as a sign of a responsible employer showing they're serious about reducing risk in the workplace. It has to be part of the safety policy and applied consistently. Once employed, testing only is allowed if it's in the employment contract and the employee's job justifies it.