Yes, scientists are unbiased when they issue dietary recommendations. After all, they are scientists. They are not lobbyists who could make a profit on certain diets, they are simply employed to study the facts. Unless someone from one of the lobbies gives a scientist money, he or she will always be unbiased.
For the most part I do not believe they are unbiased. There are so many influences that may have access to the scientists doing the recommendations. Such as a a dairy company can be pressuring them to recommend more dairy products, or same with meat industries. Or that scientist may already have his own personal opinion and use his authority to recommend what he believes instead of knows to be true.
There really is not pure science being done these days. All research is funded by large corporations. The results must suite the company's purpose or new research is done until the companies get the results they want. Politicians are also bought by these companies and ensure that not unflattering information about the companies becomes public.
Ideally scientists should always be unbiased whenever they issue a report, but the recent scandal surrounding sugar show that they are not. Companies have resorted to bribery or used in-house scientists in order to sway the findings and fudge the results in there favor. This is an unethical practice and should be stopped.