The chipset is just one variable when choosing a laptop. If a laptop has a great chipset for Linux but looks like a pile of ass and weighs five pounds, who cares what the chipset is? What about usability? For the average (non Linux zealot) user, they'll pick their laptop on the basis of portability and yes, to some degree, aesthetics. If Linux doesn't work on that laptop - and quite often, it won't - then as far as they're concerned, it doesn't work well on laptops, and they're right.
Disclaimer: I am a big fan of FreeBSD - if you think linux is bad on laptops.....
Apple hardware isn't as diverse I'd wager, and as such it's more likely that an Apple OS will be more tailored to work with Apple hardware. That and the quality build, make up the premium.
Some laptops are stamped with Linux support - something like Suse professional and they are more likely to work.
It's a bit of a gamble at the end of the day, and I'd say the same for Windows running on a laptop. If every thing just works, praise the lamas - keep it. A bit of homework certainly helps before going out and buying something.
I have OSX Tiger on an old power book. Free quality apps are few and far between. Though I like the OS. Loads of apps on Windows, but I don't like the UI.
OSX and W7 users can always just place Linux in a VM if they wanted to and have the best of both worlds. Ideology and small touches in Gnome keep me wedded to Linux.
You're quite correct, it's a known fact that Apple hardware is not nearly as diverse. This is a valid point in the carping about Windows stability in many cases - Windows has to deal with orders of magnitude more hardware than Apple does, and the difference is one of the big reasons for OS X's stability (along with the Unix underpinnings). As for the hardware vendors, it may well be their "fault" but the average user won't care - as far as they're concerned it either works or it doesn't, and at the end of the day, they're right.
Disclaimer: I am a big fan of FreeBSD - if you think linux is bad on laptops.....