I by and large agree - legal and social attitudes toward weed are hypocritical and overreaching. However, it's all not that simple. There are legal and health concerns that need addressing:
Legal - If grass or other drugs directly or indirectly jeopardize public safety (for example, DUI or committing crimes to support a habit) then that's a problem - possibly best addressed by penalizing the behavior rather than banning pot altogether.
Health - Back in the day, grass was considered non-addictive and "organic." Since then, other (real or imagined) health issues have come to the fore; most notably: Potency is many-times what it used to be. Changes in brain-chemistry and degradation of the immune system are suspect. And a whole range of psychological issues associated with pot continue to be debated.
Specific to football performance on the field & in the training room, we might want to ask ourselves the following question - "If you were about to be operated on, would you want your brain-surgeon to be sober or high?"
If I were a GM, I'd be leery of drafting a guy with a grass or other substance-abuse problem, but also believe that, as a society, our anti-weed laws are hypocritical and tantamount to killing a mosquito with an elephant gun.