I'm a developer.
I think that it's ok that a user can use the same app on all his devices.
The point probably is that you should use the app on one device at a time. This is how most software licenses works.
Personally I also have no problem when "family" members shares the same apps (for example father and son playing the same copy of a game each one on his device). But if you buy Photoshop, for example, you can't do the same. Only one copy can be used at a time.
But to answer the first question, yes, you will be able to install and play the same app on iPhone and iPad; but PROBABLY updates that makes use of iPad screen size will cost money (i.e. I think that most developers will release a new app when iPad support will be added).
I'm looking for porting my apps to iPad and use its features and this means at least 50% of the code and graphics resources needs work. I've decided to release free updates but I'm not sure that all developers will do that (especially for 1$ apps).