>It's hard to argue that it isn't an optimization, because it doesn't affect the semantics of the program

it is guaranteed in Scheme, and it affects the semantics of programs in a completely positive way.

Much of computer science is "pure" and "abstract" like mathematics. However, programmers are still taught to use loops to calculate factorial rather than recursion in order to avoid stack overflow. In Scheme you can use recursion without flinching. That is a semantic difference.