It’s easy enough to say that for free is wrong. For example on Yahoo! Answers:
Commercials, newspapers, actors, politicians, almost everywhere says “for free.” But isn’t it grammatically incorrect?
Yes, it is grammatically incorrect because “free” is not a noun. It would be correct to say, “They got in free” (used as an adverb) but not “They got in for free.”
Answers.com is more nuanced:
Is the expression for free incorrect grammar?
No, just bad for profits.
I can definitely go for that answer. I am in the business of advising people how to use English, either personally in classes or through my books. Although like most people nowadays I am a tolerant descriptivist at heart (provided that the parameters allow a common communicative standard to be maintained), I know that people who pay me good money for my advice don’t expect to be told in later life that what I have tolerated, or indeed taught actively, is ‘ungrammatical’. It is indeed bad for profits – mine as well as theirs. I also know that students, especially Spanish-speaking ones, don’t want to be told that it’s a grey area. They can handle different forms of expression in different situations; that’s obvious. But the idea that practically everybody says something but it is still widely felt to be wrong is hard for speakers of a more rigidly structured language to grasp; they just want to know what’s right and what’s wrong and who shall blame them for that? So, with such as and like, split infinitives, hopefully as a sentence adverb, for free, deferred prepositions and a few other things, caution can sometimes come close to prescription.
In my Guide to English Language Usage I try to hedge my bets. I say under hopefully:
It is also used with the meaning: It is to be hoped that. Some people object to this use but many others ﬁnd it is useful to have a way of expressing a general hope rather than a personal one. Hopefully, this space ﬂight will be as free of incident as its predecessors is different from I hope you have a pleasant ﬂight.
and for split infinitives:
… it must be said that there is still, rightly or wrongly, a considerable feeling among English speakers that a split inﬁnitive is wrong. Sometimes it seems natural to do so but a decision to split an inﬁnitive deliberately should never be taken lightly.
Having said all that, however, what is actually happening with for free? It is obvious that the answer given in the first example conforms easily and perfectly to the traditional rules of grammar, and with the alternatives of They got in free (adverb) and They got in for nothing (preposition + pronoun) there is no need, traditionalists would argue, for a form that goes against the rules.
But people disobey the rules, as we know. People do say They got in for free. I think that the explanation may lie here.
1a) How much is the entrance fee? It’s 10 euros.
1b) How much is the entrance fee? It’s free.
2a) I got in for 10 euros.
2b) I got in for nothing.
2c) I got in free.
I suggest that in I got in for free we see a confusion of 2a and 2c, following the pattern of 1a and 1b, where 10 euros and free are interchangeable. Such mental confusion is surely quite natural, and the spoken result is perfectly understandable, so I see no objection that can be made on communicative rather than formal grounds. For that reason I do not oppose the use of for free.
But there is more. I took the example of an entrance fee from the given example, but let us look at a more concrete one:
How much is that sweater?
It’s 10 euros.
What, we may ask a traditionalist grammarian, is the antecedent of the pronoun in the answer? Is it the sweater? Is the sweater 10 euros or is its price ten euros? Should the question not be correctly What’s the price of that sweater? if a grammatically logical answer is to be made? After all, in a different context the answer to the question How much is that sweater? could be 100 grams.
Finally, it is always satisfying, and almost always possible, to tie pedants up with unarguable counter-examples to their insistence. If for free is to be condemned because it consists of a preposition and an adverb, what are we to make of for ever, for good or for sure?