There are three things you have to be aware of. One, the developer gets nothing from you anymore once you paid for the app. In other words, the price you paid has to cover both initial and further investments of the app's development. Second, you don't have any claim on getting updates either. Developers need to be paid too to deliver updates, but they are getting nothing from you anymore. Third, and related, the original app had no iPad support and you accordingly never paid for it. The fact that universal apps do exist, does not give you any claim on additional features if the developer never offered those to you. If you want iPad support, you have to buy Clear+, simple.
The heart of the problem, in my judgment, is the rigidity of the App Store. Developers have no easy way to market their apps beyond the tools Apple offers. That is: no upgrade pricing and no partial pricing (e.g. to pay and get only iPhone support of a universal app). Instead developers have to make a choice between offering the update for free, raising the price, offering a completely new app, or making something work with in-app purchases. Why not give developers the flexibility they need? Who is Apple to decide how developers can market their apps?