The question is not whether it should be legal to smoke MJ, or if it is moral to smoke MJ. The question is, does it put your partners needlessly at risk (or even greater risk if the one who smokes is the sensei or a senior resposnsible for the overall safety of the class).
I would not deny the opportunity to practice to those who are prescribed Marinol, nor those who (where it is approved) used medical MJ. Nor would I want to deprive those who must take heavily sedating narcotic medications for pain. BUT...
for their own safety and that of the others on the mat, I would expect them to act responsibly and NOT get on the mat after using them. Wait until after. If unable to wait, then you are probably too ill to do more than watch right now, anyway. What are some medical indications for Marinol? Severe nausea due to chemotherapy; severe wasting due to AIDS. If your symptoms can be controlled with the drug without using it so often you need to use it before you get on the mat, fine. Otherwise, you are probably not good for training right now, due to the health reasons alone, let alone your inability to devote full concentration due to the medication effects. Not only do you put your partners at risk, but you with ill health to start with cannot afford to be foggy while training.
It is legal to drink alcohol. But who wants to ride in a car or a plane with the driver/pilot drunk.