Techokami

I assumed it was something Google forced them to do because of SEO

The thing that makes me nuts is that this is the most solved problem in the universe. Every forum ever does it! My toy blog does it!

And publish a full change history that lets us see all revisions to the article with the reasons for them

I've found that at least in some cases, the last modified date is buried in the autogenerated metadata of the page. I think Wordpress, at the least, allows you to post that information as part of their page templates, so the feature is probably there in other website software.

But that still doesn't guarantee that the date is accurate. IGN had an amazing archive and every page had their original publishing and last modified dates, but after they did a massive overhaul of all their content... the last modified was altered to the date they did the changeover, as well. :(

Edit: Yes I'm all for this info being published on the page itself in a visible way. There's no reason not to do it, especially for journalistic-type sites.