Even if it's not easy to know algorithmically, it is very easy to know a spam message when one sees the message.
News on the other hand, especially on political issues with differing opinions, are not that clear. And the underlying facts are not that major a factor either, because journalists can lie with statistics, be selective, omit things, put a false spin on the impact of real facts, etc.
And even who measures the "facts" can be political (e.g. a researcher paid by some mega-corporation, a government agency that wants to make the government look good, etc), and seemingly 100% accurate stats (e.g. on the "level of unemployment") cannot always be trusted and sometimes differ to reality by a huge margin (e.g. the level of unemployment can be manipulated in 100 different ways, based on the definition on unemployment, and various countries play all kinds of tricks with that.