Hi 
I think following thread covers this pretty well with many questions answered related to different models or modes like MAX. Have a look at the Docu links listed there or at docu itself on Models page.
in short: yes because 4.5 preview is a more costly model that OpenAI charges higher and therefore is a separate tier if someone has the need and the right task for this type of model. Cursor provides it as Usage Based only as its cost is not comparable to regular already well working premium models.
Let me know if this helped or if more info is needed.
Fellow Cursor user.