I would agree there is a problem with the public schools - and it starts early. But it's not exclusively there. I have reviewed research work from a number of universities and institutions - public and private - and from consultants to me and others, with similar problems to some degree in many if not most cases. These people are high-level academics and (highly - much more so than public employees) paid professionals, but they have similar writing problems. So it's not just the public schools. From my experience, I'd rank the likely causes as:

1) English as a second (or farther) language. Translating from another language as you write is hard.
2) Twitterization. Whether or not English is the non-primary language, the tendency these days is to just type and shoot. If no editing (or even reading once before sending) is done, egregious errors are inevitable (there's some >6th grade language!).
3) Related especially to #2, many commenters here and in other fora I use seem from their writing style to be quite young. They may not be familiar with older methods of checking their work.
4) In the paid reports I've seen, the biggest problem is lack of editing. The authors just produce a product and send it with minimal review, independent or otherwise. Editors cost money and reduce profit. I'm guilty of that too, but I usually try to reread my serious work at least once, after an hour or 2 or the next day, and make the necessary corrections (there always are some). Many consultants and researchers don't seem to do even that. When combined with #1 and #2 above, that's a recipe for amusing if not disconcerting errors.

Journalism of course is subject to all of these problems and more (get it out NOW! we don't care what it looks like - the readers will figure it out eventually if they want to). With the move away from in-house reporting to stringers, blogs, and contract outfits I would expect the trend away from readability and reliable facts to continue -- caveat reader.