The OP implies no such thing.
>Instead of buying a 20-pack of $necessary_product that costs $20 and will last for months, the lower class buy a 2-pack that costs $5 and lasts a couple weeks because they need the $15 to pay rent.
That directly implies that if they could be lent the $15, they could get the long term cheaper option and save money, even if they had to pay it back plus a high APR.