Because Linux is freer, faster, and generally, better. Also, your FUD regarding iPod support is inaccurate; Linux can support iPods (and some distros come with the neccessary codecs preinstalled), just not Apple's proprietary DRM. This is Apple's fault, and the legal workaround is to burn your iTunes library to CDs (which Apple lets you do) and then rip those CDs into Linux, or run Windows or, bwahaha, OS X, in a virtual machine, and use iTunes in it (not sure how well that'd work but I see no reason why it wouldn't...). Also, some versions of iTunes are supported by WINE.
Also, the Linux GUI is not unfriendly, nor is it 'frustrating" to config, for the simple reason that there is no single Linux GUI. KDE is the most heavily configurable GUI I've ever seen, and is also ridiculously easier to use. GNOME sucks, but is somewhat fast and n00b-friendly. The other desktop environments vary enormously, but all of them seem to be markedly faster and more intuitive than Aqua. I find the default configuration and system tools in OS X to be very counter-intuitive and unusual (coming from the standpoint of a hardened Windows power user).
I'll give you the iPod point, I wasn't aware of that, but I should have assumed someone somewhere had done it (still doesn't fix the DRM problem though, but oh well).
Freer? If I already bought a Mac, how is it freer? I'll already own a copy of OS X. Unless you mean free as in speech, but I'm not going to let that sentiment that has no effect on my work decide my OS.
I question your knowledge of UI designs. Generally, Linux GUI's tend to be remarkably inconsistent because there is no unified UI guidelines; application writers just throw whatever UI on their apps they feel like. Apple on the other hand has specific human interface guidelines that application developers have to follow for their applications to fit in, and most do.
Linux GUIs are inconsistent and unrefined, and not particularly noob-friendly. You still have to install most packages through the Terminal. Getting WINE working on my Linux box was an extremely frustrating experience.
I haven't benchmarked, but I'll just assume you're right about Linux being faster. It probably is.
Better? What defines better? Difficulty of use, inconsistent GUI, etc are superficial, but they matter. How about ease of use? It tends to be much more difficult to accomplish tasks in Linux or Windows compared to OS X and require a much higher level of technical expertise. How about stability? My Linux and OS X boxes have been pretty dang close. Available software? Windows takes the cake due to the piles of freeware crap out there; Linux has a lot of great open source projects, but OS X tends to have very high quality shareware, lots of open source, AND has the commercial apps that I
have to use.
I like Linux. But I disagree that the smart thing to do with a new Mac is to remove OS X and put Linux on it.
This is Apple's fault, and the legal workaround is to burn your iTunes library to CDs (which Apple lets you do) and then rip those CDs into Linux,
I don't have a problem with music files, I use this workaround all the time, and I rarely buy anything off of iTunes Music Store anyway (just a few Weird Al CD's

).
My problem is with the video files. I bought Season 1 of Heroes and one or two movies. No way to strip the DRM. It's not really that important; worst case I can turn off syncronization on my iPod and toss it over there manually. I just won't be able to play them under Linux.