I hear different reasons from different editors, and none of them seem to care what the others are doing with regards to that specific part. I know of a few mags that only require the writers to take the story off their site during the exclusivity period, and nobody holds that against them at all (and this is double-true if the market is a non-paying). I just know better, as someone whose income has been 85% Google SEO money for the last fifteen years. The not giving credit thing--the editors whose work/projects aren't being acknowledged will likely have strong feelings about that.