The stock taking a big hit is probably because although smoking marijuana may be legal in California, it's not something investors like to see the head of a company doing, when the company is having "issues" and that person has been acting oddly/irrationally and there are already rumours about how he may be using other, illegal substances.
For some reason a lot of investors like the companies they have large sums of money in, to be run by people who aren't openly taking mind altering substances, especially when the company isn't doing well (orders not shipping on time/qc being poor/rumours of staff being treated badly/bad or illegal working conditions), and those people are already acting oddly in very public ways.
A lot of investing in a company like Tesla requires that you trust the people running it to do a good job, unfortunately for the investors Musk's actions over the last few months have not been exactly geared to inspire confidence in him personally, or his company.