These study authors are essentially urging that further research on this policy be conducted in order to find support for the policy. What they are basically saying is: "We didn't find any evidence that the policy worked, but we are committed to the policy anyway, so future researchers should try to find evidence that it worked so that we can justify it after the fact."
Tellingly, the conclusion did not state: "Additional research aimed at determining whether or not required point-of-sale anti-smoking posters are effective in deterring smoking is necessary to evaluate whether or not this policy is justified." Instead, it promotes research in order to "provide further rationale" for implementing these policies.
Frankly, at this point, I am not so surprised that anti-tobacco advocates would have a pre-determined agenda that is devoid of an evidence base. The only surprise is that these advocates would readily admit that the purpose of research is to provide a rationale for this pre-determined agenda, rather than to figure out whether the policy works or not.
It is evident that to many anti-smoking groups, the answers to these research questions are not important. The agenda has already been determined and the point of the research is not to investigate the effectiveness of the policies, but to attempt to manufacture evidence to justify the policies post-implementation.
In other words, we already know "the answers" to the research questions. It is not the truth we are looking for, but merely evidence to support the already determined answers.
The Rest of the Story
When I entered the field of tobacco control research, I did so because there were many important questions that needed to be answered and I sought to help provide some of those answers in order to guide advocates to promulgating the most effective policies that would have the greatest effect in reducing tobacco-related morbidity and mortality. Throughout my research career in tobacco control, I have always strived to conduct research with the express purpose of determining the scientific truth and to report findings objectively. There have been numerous times when I have found "negative" results. Instead of cherry-picking around the key findings, I have simply reported the facts. It does no service to the movement and to the public to slant negative findings. If the interventions are not effective, then we don't want to be implementing them. We want to implement policies that are effective.
For example, my research on the effect of youth access laws on youth smoking showed absolutely no effect of these laws. Rather than try to slant the findings, I reported the facts and cautioned advocates not to continue spending so much money on a program that did not appear to work. Similarly, my research on the effects of smoking bans on adult smoking failed to reveal any significant effect on quitting. We reported this negative finding, rather than trying to slant it. We did not call on further research to try to overturn what we had found.
I don't exactly understand why the field has changed and why the search for the scientific truth is no longer paramount, as acknowledged by this article.