Since the wording of the question asks about the use of marijuana, many Americans could be thinking that marijuana use should be decriminalized -- that people should not go to jail if they simply use marijuana. This is different than full commercial legalization.
Gallup did not ask, "Do you think marijuana should be commercialized like alcohol and tobacco?" Both (highly regulated) industries market to children and heavy users. Is that what Americans want with marijuana, too?
The Gallup webpage about the poll concludes by stating:
It has been a long path toward majority acceptance of marijuana over the past 44 years, but Americans' support for legalization accelerated as the new millennium began. This acceptance of a substance that most people might have considered forbidden in the late 1960s and 1970s may be attributed to changing social mores and growing social acceptance. The increasing prevalence of medical marijuana as a socially acceptable way to alleviate symptoms of diseases such as arthritis, and as a way to mitigate side effects of chemotherapy, may have also contributed to Americans' growing support.
Whatever the reasons for Americans' greater acceptance of marijuana, it is likely that this momentum will spur further legalization efforts across the United States. Advocates of legalizing marijuana say taxing and regulating the drug could be financially beneficial to states and municipalities nationwide. But detractors such as law enforcement and substance abuse professionals have cited health risks including an increased heart rate, and respiratory and memory problems.
I bolded the last two sentences because they are especially relevant to substance abuse prevention advocates.
- As we know in our state, marijuana taxes are not going to be financially beneficial for municipalities. In fact, municipalities are asking the state for more money to implement and enforce the new marijuana system.
- If the message that Gallup and others are receiving from substance abuse prevention advocates is that the reason we are against legalization is because of "increased heart rate, and respiratory and memory problems", we have a lot of work to do to get our messages out to the public.