Get your facts straight.
MS e-mail apps have had a junk mail filter since, oh, Microsoft Mail & News, which was a '97 app. There's such a feature in Outlook Express and Entourage, too. All of these apps have a junk mail filter that can be told that "this message is not junk", and they also have a trigger-happiness slider, which Apple's Mail lacks.
Apple certainly didn't invent the junk mail filter. They came up with the idea of using semantics analysis to do it (ie. not looking for specific words or sentences or the sender's address but instead the format of the e-mail). In other words, Apple didn't invent junk mail filtering -- they just developed a new means to a pre-existing end.