It's a test to see how easily you are dominated. She will use all manner of words to convince you to stop pot smoking. Once you give in, this is a future green light to other 'things' about you she would like to alter. As soon as you are contolled by her, you lose some or your attractiveness in her eyes. She will say otherwise, but really she thinks you are a pussy for letting her control you.
Much like a lioness will have a bit of a fight with a lion when he seeks to mate with her, she will put up a fight to test the lion's worthiness. The lioness doesn't want to breed with just any lion, her seed donor must be dominant. If the lioness can scare a suitor away, or indeed even prevent the intentions of the male lion, then he is just not worthy. What lioness will mate with a weak male lion? None. This would produce poor stock, and any potential breeding partner must prove his mettle to demonstrate that his superior genes will produce superior offspring
Human women don't have (visible) fang and claws, so their methods of testing their mate's mettle is a slower, more inductive method.
Women are hard-wired to break men. They test by probing and using all their guile and wiles (read: sex) as a carrot. Her request has nothing whatsoever to do with your health, more to do with her reputation, and the fact you are spending money buying weed, and not on her. She would like to have an input (read: control) over how your money is used, and you buying weed is akin to you operating outside the spheres of her domain. Without her permission.
It is all about control, and using their companionship (read: sex) as ultimatum's to secure control over you and your potential. It is possible that the problem with you using pot was a pretext to enable her to present an ultimatum and walk away from her relationship with you. This is a popular technique with women who want to explore other avenues (read: different cock) without feeling like a slut, but need a justification for breaking the implied committment inherent in a relationship. Once she has got her fix of 'real men', she will initiate the forgiveness and allow you to begin the steps towards reconciliation (read: beg for forgiveness and plead obedience). The lesson you learn: Do what I say or I will let other men fuck me.
In her mind, she is meant to want you, to need you, her dominant male protector. NOT be able to control you by restricting access to her vagina. You have already proven you are a 'broken' male to her, and she only wants you back for familiarity, your earning potential, and most importantly because she can control you. She is effectively brainwashing you using sex and all the emotions you mistakenly attach to it.
You could try the 'talk it out' approach, which is basically negotiations of how much of a lackey you will be.
She will say "You don't need pot, you have me.." or somesuch other tripe.
She won't even agree to you smoking it socially, saying " You'll start again if you smoke that..." or more such other claptrap.
It's all about a method of control and dominance. She will look everyday for evidence of you smoking, and will find/store/invent any dirt she can regarding you and weed, to be used as a 'get out of jail free' card whenever she needs to re-establish the pecking order, or if she just wants a guilt-free way to take four strangers cocks at once.