"Operators tend to have a very specific vision of the image they want to project through the third-party applications they sell, if they choose to sell applications at all. Most operators are still in a content 'stone age', offering mostly ring tones and games." (Here's a link to the interesting article.)
Yes, well, I think part of the problem is that it's a much more complex situation with applications than with games or ringtones. Games and ringtones are obvious customization items: we all have different tastes, so we want different ringtones. It's not a deficiency in the device that it's lacking the perfect ringtone for me, or a game to keep me occupied while I'm on the bus. Applications, however, is a different story. If you have to buy Handy Weather for your phone, the lack of a built-in weather app is quite obvious. It's even more obvious if the application you want to sell is a better replacement for an embedded app. Why wasn't the better app already installed when I bought my phone? Right, because the operator is only there to make money. It's all about appearance.
On a more technical level, applications feel more scary. A ringtone doesn't have any functionality, it's just a sound file. A game just draws stuff to the screen. At least that's what you'd believe if you're not an engineer. An application extends and alters the phone's behaviour. Who knows what it might do? Who knows how the users might react to these changes?
And then there's the infrastructure. That an application is Symbian Signed and can be installed without warnings on a phone doesn't mean that it's actually been tested on it. It's been tested on a compatible phone, but how compatible is compatible? How much testing does our operator have to do to feel confident enough about an app to start pushing it to their users?