The mathematicians might be able to explain things  to you, here are several threads here already about the sort of thing you want to know.

Only the programmers and owners know the real reasons they do anything,but I suspect it has to do with peoples misunderstandings,and psychology.

They encourage you to do something or buy something on the expectation, it will be a great benefit, but then you find its not as good as you thought.

Regarding the tax, I think I read something about this somewhere,its to discourage multi accounting,if there was no tax,and could move resources quickly more players would do it.

The tax also takes resources out of the game.

I would have thought that if your hero is level 61 you are more likely than not, to have done all the research to get the hero moving at top speed.