When you provided a credit line from a lender, your credit worthiness has already been evaluated - hence, being granted the credit line. Why should the lender being involved in deciding what you spend that credit on, provided it wasn't a part of the original contract (eg. a car loan), and you continue to make your payments?
It was part of the original contract. A credit card isn't a completely unrestricted line of credit. You either need to use it in transactions with merchants that they have a payment processing arrangement with or use it for a cash advance that comes with extra fees to offset their increased risk.
Because risk is a time function, not static. If credit providers approved credit at one point and time and never reviewed or limited it, they would be exposing themselves.
Your example (car loans) have time limits and contracts that account for risks in the time limit - the longer the time limit, the more potential risk. Credit on the other hand is ongoing.
