The concern as I understand it is not that a journalist would just straight-up defend a conspiracy theory. Rather, the concern is that if you give any airtime to conspiracy theorists, even in the form of a debate, some viewers/readers will conclude that there must be some nugget of legitimacy in the conspiracy (why else would it be on the news?). Indeed, conspiracy theorists have relied on this to gain support, especially before they had the internet, when strategically baiting the media was the only way to get their bullshit noticed.