Sure, that may be true that scholars are more liberal than other segments of the population, but at the of end of the day, scholarship is value-neutral. It has to be, otherwise we won't build knowledge. Plus, science itself is self-correcting and very public. We move forward through replication and multiple studies (in this case, polls) are done to ensure that we achieve consistency in results. It's open so that people can challenge the status quo and the prevailing wisdom. There is really nothing better than that system we have for scientific inquiry. It is truly amazing when you think about it IMO. Just like SBR takes money from the books they review, some may see this as a bias preventing SBR from doing their job of a watchdog. Same thing here, just like most "scholars" are probably liberal, that does not prevent them from critically evaluating the presidents that have served in office. You have to take the system for what it is. That's why I said, take a poll of worst presidents according to the public with a grain of salt and believe the one's historians and scholars do. Plus, should not it be obvious that the two worst presidents are the two most recent? You think the public can think back very far?
Nobody every says there is a conservative bias in the military, in the business world, in the religious places people worship. No need to question the integrity of academics on their presidential rankings. Pretty much all academics are value-neutral. They have to be.