Clarification: over 40% of young men in the US have been arrested. (It is ambiguous whether "youths" refers to both sexes.)
(It is also possible to interpret the headline as saying that 40% of all youths worldwide have been in the US at some point, during which they were arrested, but I doubt many people were confused by that.)
From the article: "At age 18, arrest rates were 12 percent for white females and 11.8 percent and 11.9 percent for Hispanic and black females, respectively. By age 23, arrest rates were 20 percent for white females and 18 percent and 16 percent for Hispanic and black females, respectively."
The parent comment is clearly referring to the HN headline, not the article itself. Since young men do not massively outnumber young women, it is not the case that 40% of all US youths have been arrested.
The current title says "Over 40% of all young men have been arrested in the USA". This can easily be interpreted as 40% of all young men in the whole world have been arrested in the USA. Instead, it should say "Over 40% of all young men in the USA have been arrested".
The actual article title: Study: Half of black males, 40 percent of white males arrested by age 23 seems perfectly adequate ehre. I think HN readers can see the big underlying issue rather than being sidetracked by the racial disparity (which is also important, but not the main story).
(It is also possible to interpret the headline as saying that 40% of all youths worldwide have been in the US at some point, during which they were arrested, but I doubt many people were confused by that.)