Perhaps they're basically preventing people from being disappointed when their older impotent PPC-based machine craps out and buckles under the weight of today's increased computing demands?
Just to echo a comment above, it's not like Apple hasn't given everyone plenty of advance notice that the PPC-based chips aren't going to get them where they're wanting to take the platform (and to keep up with PC competitors!), and they were going to migrate (read phase-out support) to the stronger Intel-based chips. Hardly news to anyone who's followed the keynotes, etc.
I've owned Macs from the Apple IIe days, and have owned Apple laptops from PB3400 series: while I don't like that my 1.5GHz Alum G4 Powerbook is considered dated and now a 'legacy' machine, it's not like Apple is reasonably expected to ship me a retrofit Intel CPU designed for g4 Powerbooks, just so it can keep up with the rest of the line! (If it were even possible!) No, It was time for the machine to be put out to pasture and retire for lighter uses (surfing, e-mail, etc), and not hold up the rest of the product line.
I've long ago updated to a MBP 2.6GHz Dual-Core, and the thing puts the ol' Powerbook to shame (like comparing a .357 to a pea-shooter). Truly a significant upgrade, and you only have to use an Intel-based machine (watching Activity Monitor) under a heavy computational load or with video demands placed on it to see WHY they NEEDED to move on. For example, I can run video apps (e.g. EyeTV) on my Intel MM that will stumble and fail on an older PPC MM. It's as simple as that.
Now, where I call Apple out is in not knowing which machines support iTunes 10.0.1.
I recently reinstalled OS X.10.4.11 on the 1.5GHz G4 PowerBook, with plans go no higher than 10.4.11 (since I have outdated software that cannot run on anything higher, unless I PAY for the 3rd-party developer for an SL update. No thank you: your software works fine under 10.4.11). I'm letting my GF use my machine, with the idea that I can borrow it when I rarely need to run the legacy disk utility software (maybe 1x per year: it's software that re-partitions a drive without re-formatting).
So after going thru a clean install to X.10.4.11 and the lengthy set-up (i.e. updating via software update), I fire up iTunes 4 (installed via OS X.10.4 DVD), and a pop-up appears which tells me iTunes 10.0.1 is available for download.
Now, I consider this WRONG: Apple's servers SHOULD know I'm using a PPC machine, and SHOULD know that 10.0.1 will refuse to install on my laptop; don't waste my bandwidth and time prompting me to install the wrong software! This happened LAST NIGHT, so it's still an issue....
As it happens, after it failed to install, I knew to check Apple's site, and saw the older iTunes software (8.2.1) that was recommended on the download page; I had success with that.
HOWEVER, I see that a version of iTunes 9.2.1 is no doubt a better choice (as linked above). So it's back to the download section....
Come on, Apple: while I don't expect you to perform miracles by making my PPC as powerful as a new Intel-based machine, at least fix your servers and don't mislead me and waste my time by directing me to an incorrect download option (whether 10.0.1, or 8.2)! I dare say it's not asking too much for you to know your own legacy machine's capabilities, and prompt users to the proper version.