If your employer has it written into your contract (which you signed) and then enforces drug testing then yes they can
But that's what I mean; is it right that the NRL can demand total abstinence from recreational drugs in the off-season? It's a bit unfair, I reckon. And I wonder how the players' association feels about it? (We wouldn't know because they hardly ever say anything about anything.) Yes, we can all take the moral high ground and say that nobody should be snorting coke. But how many of us would still be employed if we worked under the same Stalinist conditions that NRL players do?