a crux of this argument is that Apple considers macs and mac os x to be one "product" or "entity." the only reason they are sold separately is to update older computers. you don't see anyone clammoring for Apple to sell their iPod software for other mP3 players or the iPhone software for other smartphones. the only reason there is an issue at all is because Apple sells Mac OS X separately. again, the only reason for this is to update older computers. it is still a closed system, and you cant separate the two, just like you can't separate the iPod/iPhone and its software.
First this wouldn't be an issue if Apple hadn't dropped the low and mid-range PowerMacs from its lineup in favor of an expanded range of iMacs in addition to having other holes. I think buying from Apple is the first choice of everyone where but when it becomes a matter of going way over budget (MacPro) or not having your needs entirely met (iMac), you start to look at other options.
Second, the iPod and iPhone are embedded platforms. I can't conduct my business interests and hobbies on one. The lack of power and memeory compared to a computer and much smaller screen also dictates the OS be highly specialized for them. My iMac is an Intel core 2 Duo connected to a Intel PM965 north bridge and ICH8M south bridge, broadcom Wireless-N, western digital hard drive, Pioneer CD Burner and Radeon Mobility HD 2600. The only things that differentiates it from about 5,000 BIOS-based windows computers with the same exact configuration are the EFI ROM chip and the casing.