All people are biased. And I know that by bringing that up, I sound like I shouldn't be supporting science advocacy, but here me out. All people are biased: politicians, teachers, janitors, policymakers, accountants, scientists. You can't ever escape bias, no matter hard you try, because it's what makes you unique. We learn bias from our families, friends, and environments; they are impossible to avoid. Since everyone is biased, wouldn't it be better to have someone involved who knows the science behind the policy, like a scientist, rather than just policymakers, who might not take the time or have the capacity to completely understand the results of any testing that was involved.
Additionally, it would be hard for a scientist to present corrupt results in a paper. Papers are reviewed multiple times in a double-blind edit by the scientist's peers. If their experiment, results, or conclusions don't make sense, the reviewer recommends that the scientist conduct more research and that the journal not publish the article. While scientists can be biased, science itself cannot. Results don't lie, and if science is done properly, the experiment leads to a logical conclusion. If a scientist tried to present something to their peers that wasn't completely supported by the results of their research, then the conclusions they made in that paper wouldn't be accepted. So rather than barring a scientist from policy making, perhaps policymakers should consider their own biases before more carefully before the scientists they work with.
Rice, J. C. (2011). Advocacy science and fisheries decision-making. ICES Journal Of Marine Science / Journal Du Conseil, 68(10), 2007-2012.