I'm with you, it has nothing to do with tax breaks. Health insurance rates possibly but I'm not sure that is even the case.
Do you even business Bro?
It completely depends on the company, although most silicon Valley companies do not test. Google, Facebook, Uber, and NCR do NOT test, whereas IBM and Amazon DO test.
Tech companies in general tend to hire millennials and that age bracket tends to be pro weed, or at least pro choice for weed. (68% support legalizing it if you believe this website)
http://www.pewresearch.org/fact-tank/2015/04/14/6-facts-about-marijuana/
Guess my definition of tech is s a little different. Our company works with the Fortune 50/100/500, financial, stock brokers, top tier retail, health & life sciences companies, etc. I can guarantee you that weed is 100% forbidden, at least on a visible level and everyone is regularly tested, often just to move into a project.