Islamic banks do that as they hide the actual interest rate they charge (sorry not interest as in their jargon they call it 'profit' rate).
However whether it's an Islamic or a Commercial bank, never make up your mind on the rate the sales person quote, always ask them details about that rate, is it the simple rate or the IRR, which is the actual rate that you will be paying.
Rather than relying solely on bank's word, better is to always calculate the IRR yourself. All you need is the fixed installment amount that you need to pay each month, the loan amount and the tenor of loan (in months). Put it in MS Excel by using the 'Rate' function and you will get the monthly IRR that multiplied by 12 would give you the actual rate charged by bank.
Also I can tell you there's no point in complaining to Banking Mohtasib or legislation. But yes you can just lodge your complaint of miscommitment made by DIB in SBP website. All banks fear SBP a lot and even just this threat may make the bank change a lot of things in your favor.