Have you been taught to never allow for free? That it's too wordy, the for isn't necesssary? Omit needless words? Read this piece from Arnold Zwicky at Language Log.
His best argument against automatically replacing for free with just free or without charge or for nothing is that for free often sounds better. Consider: I shoveled the neighbors' snow free vs. I shoveled the neighbors' snow for free. For free is much better.
Some people argue that for free isn't just wordy; it's wrong -- because it combines a preposition and an adjective. Zwicky comes up with plenty of other examples of prepositions and adjectives that no one rails against (for sure, in short). And then he plays the trump card: It's an idiom. Even if there were rules for free was breaking, it gets a pass. Idioms get to break rules.
Does this mean you should always leave for free? Not necessarily; there may be times when plain old free sounds better. But no more knee-jerk deleting just because a journalism teacher once told you to.