The same goes for credit data; you (largely) can't run a credit check on me without my permission.
This is an HR service to ease the burden of legitimate requests for employment or salary data that you, as an employee, request.
More logically, employers wouldn't want other companies to be able to access their payroll information for competitive reasons.
I can't explain your previous experience; perhaps you were at a company with firm pay bands, and they knew you were already at the top of your current one?