And I'm not convinced that they should even try.
I agree. I know that we are possibly making a bigger deal of this than needs to be made, especially when spelling and grammar is only one small part of a much larger pie, but why not let people decide for themselves if the content is any good? Sure, if a site really isn't about blue widgets, I don't want to be sent there because the site owner gamed his way into the SERPS. But really, I don't want Google deciding for me what is well-written, good, copy.
While spelling and grammar checkers have gotten infinitely better since they first emerged, there are some things that machines still can't really do all that well. I forget the author's name, but he wooed the world with his literary genius a few years ago, but didn't really like using periods. However, his books sold very well if I recall correctly. In a Google world, he might never have seen the light of day.
Funny aside, I googled "who is the author who doesn't like periods" and mostly what I got was pages about menstruation and pages about guys liking or not liking to have sex with women on their periods, and then, weirdly I got some Facebook page of some guy who hates writer James Patterson, who I didn't know because I don't read popular fiction. When I changed the query to "who is the author who doesn't like using periods" I got more grammar-related pages, but still never found the name of the writer I was looking for.
While I applaud Google's attempt to present searchers with well-written copy in a landscape where Amazon is letting any hillbilly with with a modem self publish their books -- there's a reason no publisher wanted your book, it wasn't very good -- there are still some things that a machine can't do well, and only the human eye and mind can decide if it's good or not.