It should come as a surprise to no one to learn that magazines often do less or even no copy editing on their web material. (And this isn't limited to magazines.) I don't know why companies do this.
The consistency of this practice would seem to indicate that publications are more interested in preserving a well-edited print record instead of ensuring the web version is clean. It wouldn't seem to be just money, otherwise publications could shift people from print to web.
So it would seem to be something about how they view the value of the web, and a prevailing sense that web copy can quickly be fixed, with no one the wiser. And that it is often writers, not editors, who have established sites.