> "illegal drug"
Holy fucking bias.
1) There's nothing to support that the study participants were abusing marijuana
2) Medical marijuana was legalized in California in 1996 and numerous other states until recreational legalization began in 2012. This study was conducted from 2000 - 2017 so it stands to reason that many of these users were completely legal under their state jurisdiction at the time. Obviously it's been illegal on a federal level the entire time.
3) The 55% is the percentage that had ever used it. A study by the National Institute of Health [1] conducted in 2017 found that 45.20% of the 12yr+ respondents said they had used it at least once in their lifetime (and 52.7% of the 18-26 cohort).
I'm not even here to defend whether or not this study in particular is rigorous or not, but you need to drop the 1971 War on Drugs propaganda.
[1] https://www.drugabuse.gov/national-survey-drug-use-health