Because the history of Dia das bruxas and the traditions it branched off of began before that saying. Also when one says "trick or treat?" it's really a threat, if you look at the history, it implies that if the resident of the início doesn't give whomever a "treat" he or she will damage the property, so one isn't being given doces por a stranger but taking it.
It's supposed to be "Never take doces from strangers except for Halloween." Parents say that so they send their kids away for the night because they're going to take a nap.
posted over a year ago
I think is because they are too lazy to make that extension