I also don't really buy the it's my phone argument - well yes it is, but you buy it as-is. Fair enough - it can do more - but then thats true of a lot of products. People can want it, but should not expect it.
Games consoles could be theoretically used as PCs (indeed the PS3 officially allows this) but no one really cares about this feature because people don't buy consoles as PCs. People buy consoles to run games, just like DVD players play films and toasters make toast. The Wii doesn't have an HDD, it can't play DVD videos, and is generally the most locked-down console, yet it has outsold its rivals put together.
This leads to the crucial question:
What is a smartphone? Is it a pocket-sized PC that can be used for general computing tasks, or is it more of a single-purpose device like a games console or DVD player?
In truth it's a mixture of both, people buy phones for their built-in functions (calls, photos, email etc) but there is increasing demand for add-on functions which require native applications.
This may be why Nokia's doing their internet tablets as a low key alternative to Symbian. The tablets (which use Maemo Linux) are pocket-sized PC-style open platforms which let you do pretty much anything you want including booting from completely different OSes (some people have even managed to install older versions of Windows and Mac OS). It's really meant for those who want as much access to their device's functions as possible and hacks are not only allowed but encouraged and even advertised on Nokia's official tablet sites.
The thing is, most smartphone and computer users don't want that kind of access, they prefer having locked-down devices designed around particular functions.
Linux is famous for being open, but the Linux-based EEE PC is very locked down, with no interface-based method for installing any extra software at all, yet it's sold in far larger numbers than most Linux computers. That's perhaps an extreme example, installing your own software can be very useful even for novice users, but it illustrates how the mass market's priorities are totally different to the hardcore user's.
Even in the smartphone world, the iPhone launched successfully without the ability to install any native apps, and Symbian 9's binary break meant the first S60 3rd Edition devices suffered a severe lack of native software yet they sold even better than S60 2nd Edition devices. Restrictions on native software don't seem to affect a smartphone's sales, and sales are of course the bottom line for smartphone manufacturers.