Actually, this is not a design flaw, and it is not because of internals or performance.
It comes simply from the fact that functions in Python are first-class objects, and not only a piece of code.
As soon as you think of it this way, then it completely makes sense: a function is an object being evaluated on its definition; default parameters are kind of “member data” and therefore their state may change from one call to the other – exactly as in any other object.
In any case, Effbot has a very nice explanation of the reasons for this behavior in Default Parameter Values in Python.
I found it very clear, and I really suggest reading it for a better knowledge of how function objects work.