Seems strange that PCs of equal spec to those lower spec Macs are excluded.
Arbitrarily forcing users of older Windows PC hardware still capable of running the software to upgrade despite still being capable of running the software.
That seems like something only Apple would be accused of doing.
It is due to resources management. OS X is a way more light OS than Windows and it is engineered to utilise CPU/Memory at 100% as Apple does both Hardware and Software in-house, unlike Windows cause of the different OEM brands out there the OS itself isn't optimised enough.
A good example to prove this is Adobe Photoshop, in my Macbook it just runs within 5-10 secs, in a gaming top-notch (better specced) laptop of a friend of mine, needs at list 30-45secs.
Actually a Mac can last you 3-4 years of extensive usage without the need of upgrading to a newer machine, while a Windows computer will probably need an upgrade in 1-2 years if you want to continue gaming or using resources heavy applications at ease.
It is actually really funny how iHaters call Macs expensive, when similar specced computers actually cost the pretty much the same -if not for some brands a little higher! Or people complain about Apple's annual upgrade when other computer manufactures upgrade their product lines 2 times a year or games such as Crysis are badly optimised to force people into upgrading.