The issue with double spacing after . ? ! is with the typeface. On older typewriters they use what is called a monospace typeface. Monospace typefaces take up the same amount of space no matter what character is typed, so the I and W take up the same width. Nowadays with computers, the typefaces are variable space. The . ? ! characters on variable space typefaces supposedly have a bit of space built onto the end. It isn't a full space, more like 1/4-1/2 a space. Since it has space built in, it only needs 1 space after it.
Web Browsers follow and force this rule on people. It seems to be properly using my spaces here on this message board, but in many cases on web pages, I'll type out two spaces, and it'll get shrunk to one space as the browser throws away the extra whitespace.
I didn't realize all this till about 5 months ago when one of our new hires pointed it out to me. I then googled it and found out he was right. The correct way now is to use one space, not two. I still use two spaces, as I think it provides additional clarity even with the variable space typefaces. It is mostly an issue when getting involved with publishing. They'll be very specific about what you need to use. So it will probably be something that will come up for those on Law Review and the other journals.