LEDs have suicidal tendencies. Allow them to draw more current and they will keep drawing more and more until they pop. It's called thermal runaway. You have to limit the current, either with a resistor or with a constant current circuit.
The formula you used above is Ohm's Law. When he invented that law, there were no semiconductors. He thought the law applied to all conductors, but now we know better. So why do we use the law to calculate resistance of LEDs? We don't. When you use Ohm's Law like that, you are applying the law to the series resistor, not the led itself. Resistors obey the law. Because the resistor is in series with the led, the resistor limits the current for both.