That's a good question, but unfortunately I'm not privy to Apple's internal workings to give you an accurate answer. The general metric seems to be "Does it add value to the core product?" This is the same reason iOS updates are free to the iPhone but not to the iPod Touch — the iPhone is viewed as a multi-year investment (and reflected as such in Apple's accounting), while the iPod Touch is a shrinkwrap product that Apple doesn't believe it can add value to without charging for it.
But the precise bar for when they feel like they need to charge is something only Apple could tell you, and maybe not even them. Like I said, their interpretation of the law is idiosyncratic.
Thinking about it further, I'd bet the distinction is that iTunes is on Windows too, so it's viewed as an independent product, while things like Xcode and FaceTime are tied to Mac OS.
I read something somewhere (can't remember where) that Apple is starting to spread the sale of new macs (starting with machines with Lion on them) over some longer period of time.
This would apparently set things up for providing new features (ostensibly iCloud) without having to charge small increments and still adhere to their conservative SOX interpretation.
But the precise bar for when they feel like they need to charge is something only Apple could tell you, and maybe not even them. Like I said, their interpretation of the law is idiosyncratic.