First of all you need to separate UI Design from application code. These UI design elements should be completely independent of how the app works. Second no one is saying they couldn't release an app for one platform and then update to support the other. As a matter of fact I would even support they do so if they are in a financial position to need to do so.
The UI elements can be dependent or independent of the code but that is determine by the nature of the app itself. Also sometimes tablet apps have added features to take advantage of screen space so additional coding is required. Games need additional assets and artwork for higher resolution scenarios and adjustments to screen sizes. Either way this does take extra time and resources to do so. Also right now if you want to support Gingerbread versus newer devices, you have to approach screen sizing in two different formats.
But with that said I would seriously have a problem believing anyone that says it is better to do something halfway and not look at the whole picture just to get something out the door.
It all comes down to time and money. If you really think it works the way you hope it would, why do many apps make their initial entry in the Apple App store before the Android Market. Shouldn't they plan an initial release on all platforms. Some do, not all of them. It all comes down to budget and what depending on the situation, what their investors told them to do.
I can very well understand Splashtop having two separate platforms back with there gen1 app was released. At the time it was released the only android tab out was the Xoom and that is why I bought Splasthop HD. But that is now years ago, tablets are not new on the scene now, and the android platform has evolved allot since Honeycomb.
Platform is alot better, my point was that Splashtop started out with 10 million dollars in funding. That gives them a development advantage others may not of have. If they didn't have that funding or less funding maybe they would have delayed their entry into the Android market.
If wouldn't believe anyone that says it is better to maintain 2 support paths over one, or would I really want to support an app that promotes that route simply for monetization.
In the current status of Android (and really what it has been for some time) there is no reason that someone writing a app couldn't design a good UI for both tablets and phones into there app as long as they thought about it from the beginning. One design path, one maintenance path, one application.
Most of these people are supporting multiple paths already by having their apps on multiple markets. The question is what is the fastest easiest point of entry for success.
With all that said though, you really didn't state anything that says we need tablet specific apps. If anything the only thing I saw was you were advocating for it so developers can charge for their app twice.
No my point is because of the low price selling of the marketplace, they use this as a means to gain additional funds. If apps were selling at a higher price range, maybe this wouldn't be the case. Also for certain apps that have specific functions it is harder to have buy in purchases. Freemiun games are different. Candy Crush makes around 600k from the Apple App store alone from in app purchases daily.