Is The Term "For Free" Grammatically Acceptable?
Strict grammarians assert that the term "for free" is grammatically incorrect. According to them, the example below is wrong:
Music critics get their records for free so their opinions usually don't matter.

The Reasoning
Strict grammarians state that "for" is a shortened version of "in exchange for", and "free" is a shortened version of "free of charge". So, if both were expanded to their full versions, we would have "in exchange for free of charge", which is nonsensical.

Use "For Nothing" or Just "Free"
If we accept that "for" means "in exchange for", then "for nothing" or just "free" would be the correct way to write that something is free. We believe it is too harsh to announce that "for free" is grammatically incorrect. Over time, language changes, and it's pretty clear that "for free" is widely used and understood. If you're writing for some real grammar pedants, then try to avoid the term "for free". If you're not, then just go ahead and use it.