Microsoft had a monopoly on OSs for PCs, so they were not allowed to use that to give another piece of software an advantage. They did not simply have a monopoly on Windows users.
In order to be regulated under Anti-trust, Apple would need to have a monopoly on some market that isn't simply described as "their customers". The closest they came was iPod, but it looks like that hovered around 75%
Microsoft didn't actually have a monopoly; they got dinged for trying to create a monopoly by strong-arming OEMS that wanted to sell Windows with their PCs, but there were plenty of non-Windows-centric manufacturers (Apple, Sun, SGI, Be, etc.) and plenty of alternative operating systems, even for IBM-compatibles (one of which is now dominant in pretty much every realm other than desktop/laptop PCs).
In order to be regulated under Anti-trust, Apple would need to have a monopoly on some market that isn't simply described as "their customers". The closest they came was iPod, but it looks like that hovered around 75%
http://stratechery.com/2010/apple-innovators-dilemma/
So, would probably not fall under anti-trust.