Borrowers are fully functioning adults
Well you'd think, but there doesn't seem to be a shortage of people who claim that they didn't understand what the words 'interest only' meant, despite them signing paperwork that would have included a reference to 'the repayments are for the interest only, separate arrangements will need to be made by the borrower to repay the capital' or words to that effect, plus a numerical illustration.
Eg 300 payments of £200 pm interest plus a final payment after 25 years of £50k, or whatever. APR X%.
Banks have fucked up a lot over the years (endowments, PPI, packaged bank accounts, lowering monthly credit card payments so the minimum payment doesn't cover the interest so people don't realise there is a problem until they are tens of thousands in debt), but with IO mortgages, the majority of borrowers really have to take a hard look at their own level of personal responsibility on this one.