Personally the likely hood of contaminated meat is very small, NZ's beef industry would be very similar to ours, and clem would be all but nonexistent if all present. It is after all illegal.
They would have to be more then stupid to take clem, if they did this 4yr ban never play again.
The most likely scenario is they knowingly took ice/cocaine that was laced with clem. They deserve the 4yr ban for this as well, and not just a 1st/2nd strike for illicit drug use. If this is how they got the drug in their systems I look it a bit like running a red light. If caught you get a fine, however if you crash into another car and kill someone, you get done for the higher crime. You have to pay the price for your actions and if they took clem as a result of taking another illegal drug its just to bad, they took a risk and paid the price.
If there drink was spiked, well this is unlikely and even harder to prove.