"fecal matter" is used later in the article. They probably just used "poop" in the title to catch more eyes.

Astute observation. Much care is taken when writing headlines and in the case of news organizations with print editions in addition to their web presence, the web headline isn't necessarily the same as what runs in print because of different concerns, e.g. SEO vs journalistic standards.

It was a "Poop Culture" article.

And more clicks.

I'm not saying they should - my point was that you shouldn't expect "scientific" terminology and language from the Washington Post.