A survey alone would be collective opinions, yes, but of course that's not all the researchers did. They performed statistical analysis to determine which arguments Americans tended to find more persuasive, and what factors increased or decreased likelihood of supporting legalization.

Here's a portion of the abstract that demonstrates that:

Ordered logit regression assessed the relationship between perceived argument strength and public support for recreational marijuana legalization. Respondents rated pro-legalization arguments highlighting beneficial economic and criminal justice consequences as more persuasive than anti-legalization arguments emphasizing adverse public health effects. Respondents were more likely to agree with arguments highlighting legalization's potential to increase tax revenue (63.9%) and reduce prison overcrowding (62.8%) than arguments emphasizing negative consequences on motor vehicle crashes (51.8%) and youth health (49.6%). The highest rated anti-legalization arguments highlighted the conflict between state and federal marijuana laws (63.0%) and asserted that legalization will fail to eliminate the black market (57.2%). Respondents who endorsed pro-legalization economic and criminal justice arguments were more likely than other respondents to support legalization. Our findings indicate that, on both side of the recreational marijuana legalization debate, there are arguments that resonate with the American public. However, public health risk messages were viewed as less compelling than pro-legalization economic and criminal justice-oriented arguments.