If you have two IB flights on the same PNR, of course IB will put you on the next flight if you're late.

However if someone has two flights that are on different carriers, and they bought a nonrefundable/nonchangeable ticket from IB, and miss the non-refundable/non-changeable flight then IB or any other airline are completely correct in saying they need to buy a new ticket. The reason those non-refundable/non-changeable tickets are so cheap is because they are non-refundable/non-changeable.