Then this kind of code would have blown in your face.

Besides the function being re-evaluated, the hash gets a NEW value "value" for that key. Nobody ensures that "value" is the same as the one it was already there.

E.g

var cachedReturnValues = {key: 0};

var value = 50;

if (!cachedReturnValues[key]) { cachedReturnValues[key] = value; }

So, instead of setting the value when the key is not already in the hash, now we also CHANGE it in all these false-positive cases.

Furthermore, even if (a) the function had no side-effects, and (b) by some magic the value was the same with what was already in the hash, e.g if your wrong "merely re-evaluation" assumption was true, who would ensure that the overall block has no side-effects?

if (!cachedReturnValues[key]) { cachedReturnValues[key] = value; doSomethingElse(); }

Just because we see it written in a way that doesn't do much harm now, doesn't mean it's not extremely dangerous to blow up in the future, e.g. when some naive programmer adds another statement to the block.