Americans are the weirdest people on the planet, and this once again confirms this hypothesis to me. I’m German and I’ve never ever seen anyone do this systemically. In fact, I just did a Google search in German, and all I could find was an article about plenking (unfortunately, that’s rather common here), in which the author noted that they couldn’t even find any information on double spaces after periods.
Then again, maybe nobody here does that because nobody here learned to write on keyboards in elementary school. That’s something that most people in my generation (1985) had to learn outside of school. In fact, I remember in fifth grade having to stay in school longer and learn to write prettier — by hand, of course!
Anyway, a double space after a period doesn’t make any sense whatsoever. It’s completely redundant, because the single space already marks the beginning of a new sentence and visually separates the sentences. A double space would only destroy the text flow.