There's a number of ways to look at this.
One, Apple hardware has a tendency to last a long time. My two main computers are 8 and 10 years old, respectively. At least for now, they do everything I need. However, they do require "period-appropriate" software to be expedient, which is to say they run Tiger. Now, by any account, Tiger isn't that old an OS. It's almost five, which is a little more than half of the age of Windows XP, which is still fully supported, even by Apple. And yet, Apple can't wait to pull the plug on 10.4, and when that's over (iTunes will move on, QuickTime already has, and Safari 4 will only be maintained so long), they'll be coming for Leopard next. I do not see any reason to be so aggressive in their push for people to upgrade.
Two, they don't make it easy to keep it legal. They've already made their development costs and then some back on Tiger and Leopard, so I see no reason to not continue to offer legit copies of the software to those who need it and are willing to pay, but Apple refuses to do so. This is what we call entrapment. On one hand, you cannot acquire the software from the manufacturer, and on the other, if you do so through illicit means, you're breaking the law, thus the only legal option is to continually upgrade your computer to accept the latest Apple software. Yeah, you can go and buy Tiger or Leopard on any number of second-hand sites, but even that is grey-market dealings, as the EULA for OS X explicitly says the software is not for resale.
Third and getting back to the end of my first point, this aggressive push to upgrade prior to it being necessary. Windows 7 can be installed on pretty much any five year old computer with sufficient memory (they all have sufficient processors for the most part), and while not rocket ships, they'll handle the basics quite nicely. Where is Apple's support for five year old hardware? When Snow Leopard was dropped on the market, the newest PPC machines where under four years old, and they're already out to pasture and not privy to the latest software. If Apple didn't try so hard to pry people's wallets open before it was necessary, you wouldn't have as many people trying to obtain OSX in an illicit fashion.