I think it's just a question of culture. HTML applications - although you're talking about intranet, I'm thinking more about internet - need to be usable by people visiting, people who have come across the application for the first time. If people needed training to use web sites, then the idea of the web would quickly fall apart. I would suggest that it is this cultural idea that you're being affected by; that users should be able to "discover" web apps. Whether or not the app is on the internet or an intranet is probably immaterial; the feeling is that of the internet.
One other difference is that traditional apps tended to perform tasks that were new, or did things in new ways. Web apps tend to be more aimed at supporting existing processes often, and the user would come to the app with a pre-defined expectation of the task. People shopping on an e-commerce site, for example, would have the idea of a cart and a checkout. Intranet users may be in a similar position for many of the tasks they wish to perform too. Again, this is kind of a cultural thing.
I agree with you, to the extent that the UI of the application is likely to make little difference to the training need of the user (by that, I'm talking about technology, not design - design is clearly extremely important), so the fact that one app is coded in W32 Forms and another in HTML doesn't mean the HTML one doesn't require training. But, there is a difference there, in culture and expectation.