Does Sinclair have the right to publish opinion/viewpoint pieces on its stations and have its employees tow the views of headquarters?

Or are they breaking the law or FCC regulations by publishing politically biased infomercials on local news stations and compelling employees to air things against their will and consciences?

If your response is along the lines of "It's not illegal, but it's wrong!" my questions are:

-Is Sinclair the only news source with politically biased coverage in the country?

-If not, what makes it wrong and punishable for them to publish bias but not for other organizations to do the same?