The problem is not that this type of thing exist, but that whom is targeted to. It is targeted to those it can attempt to take advantage of, and while it may be legal to take advantage of such people, it is in my own view not that different from getting a child to agree to some loan without fully understanding it. Especially since the child may be signing the loan mere to have money, while the adult who takes a payday loan is doing so because they honestly need (or honestly believe they need) the money.
At the heart of the issue lies the question, can one enter into a wrongful business dealing as long as all other parties enter willingly? For example, a millionaire who is dying of thirst in the desert, is it wrong to only offer him your water for half his fortune?
For me, the answer is yes due to the existence of the child. The fact that some entity cannot entered into business dealings due to circumstances invalidating their consent means I must be open for other circumstances. In the case of the millionaire, his consent to enter the business dealing would seem to be invalidated. Now, as the exact standard by which consent is invalidated... I'm still working on it.