Why wouldn't they have both have exams? Methods Non-CAS and Methods (CAS) were technically two different subjects.
You use a CAS calculator in CAS (as the name suggests). Methods Non-CAS no longer exists, hence why there isn't a 2010 exam.
That's why they both have their own set of exams (I think they were identical though). Of course, the scaling and stuff would be different. They couldn't let some students use a CAS and then some students not use a CAS and then scale them exactly the same. I suspect that would be one of the reasons to make them two different subjects.
The other reason I can think of would have been to slowly phase out Non-CAS. It'd be easier and smoother to go slowly from non-CAS to CAS. Probably wouldn't have been smooth if VCAA came out in 2006 and said that everybody has to use CAS.
There'd be other reasons, but it makes sense for it to be considered two different subjects.
edit: fixed typo