It's because not all machines are capable of running 64bit Kernels.
http://arstechnica.com/apple/news/2...ard-build-limits-most-macs-to-32-bit-mode.ars
In order to run true 64bit (apps & kernel) you need a recent machine, and you still have to hold "6" and "4" while booting.
Not being able to install 64bit Windows on a Mac has nothing to do with Apple. Unless of course you are referring to drivers / lack of written by Apple.
I had Vista Ultimate 64 running on a machine with all functionality.
It's not that entirely correct..
Snow Leopard, like Leopard, can execute 64bit applications. Those apps run with true 64 addressable space - I've compiled 64bit apps with 3.2GB of memory used on Leopard.
More of the Apple applications are 64 bit in SL. Such as Finder etc. I'd suspect more of the libraries behind those are 64 too.
The kernel defaults to 32 bit as most drivers aren't written - things like Parallels for example may not work with 64 bit kernel.
I have a feeling my SiliconImage SATA PCI ExpressCard isn't going to work for example..
However one point that no one has mentioned yet - security. The 64 bit more provides a better security mode whereas if the system defaults to 32bit then I suspect that the security features are also going to be disabled.
My MBP is capable of 64 bit (Santa Rosa). My EFI firmware is ok with 64 bit. So I'll be a bit peeved if I'm limited to 64 bit without an option to default to 64 bit instead.
Ignore my ZFS rant.. still peeved. Only available on Server.