Yes, Donahue was presenting opinion, but opinion was infecting regular news broadcasts long before Donahue got sacked, and that was much more worrisome than anything done to talking heads.
Television news impartiality died long ago, with the Reagan administration's gutting of the Fairness Doctrine. Not only did talk radio do away with any duty at all in using the public airwaves to present opposing points of view (the rise of Rush Limbaugh is a direct result of the end of the Fairness Doctrine), but network news gradually dispensed with any notion of impartiality. The ultimate outrage was Rupert Murdoch's entry into the American news market with his blatantly propagandistic "Fox News Channel." He brought to America the British form of reporting (although he was Australian himself) that always presented a certain point of view, the opposite of what traditional American news organizations which at least had as their goal fairness. The British media would editorialize everywhere in its coverage, whereas American news organizations left opinion on the opinion page in newspapers or commentary segments on television that were clearly marked as such (examples for television would be Eric Severeid's or Howard K. Smith's commentaries). Murdoch, however, went further than anybody when he put a political operative, Roger Ailes, in charge of his news network. This was nothing short of an attempt to buy and shape opinion by creating a propaganda organization.
This would NEVER have happened when the Fairness Doctrine was in place.
We have been suffering from the effects of propaganda ever since.