A lot of people see marihuana not really as a drug. Many have the opinion, that it has few to no risks and therefore is rather harmless. If I remember correctly Mulligan only dealt in marihuana with the occasional shroom (shrooms could have been for private use only, don't remember that well). So marihuana users tend to think of themselves as non-drug-users and share the opinion of those on the "hard drugs". Then you need to remember the history between those two. Patrick beeing on crack (or was it coke?) while on the tour escalated things badly, so it is no wonder Mulligan picks on him in that manner.
The stated opinion on marihuana differs from mine.
Next on the list
We might be looking at suicide #2 coming up soon. Clusters are known to happen. Or, he could OD, an inevtiable hazard when one's addiction is a street drug with typically lousy quality control...