Obligations are bought and sold. If nobody buys them, then the Greeks do not get the loans they wanted (and had no ways of repaying with their phoney budgets).
If the value of the loan goes up (meaning it costs more to repay it) then the interest similarly falls, and reversely, if the value drops, the interest rises. There is nothing immoral in that, in itself.

What is immoral is that bankers gave loans to the Greeks, knowing full well that the Greeks were probably insolvent, calculating that all the risk would be absorbed by the EU. They bought high-interest obligations from the Greeks, and reaped the benefits of that until the Greeks began having trouble paying, and then they expected the EU to pay out their bonds.