I strolled away from Google Plus to visit Twitter a few moments ago, and I pointed out that “for free” is criticized because “free” often works better in half the space. Usage guru Bill Walsh of the Washington Post pointed out that the real criticism is that “free” is not a noun, a more challenging argument.
The idiom forces “free” into the position of a noun, as if it is the same as “zero dollars” or “no pay.” It’s hard to reconcile, so it might always be considered nonstandard, or as Webster’s Dictionary of English Usage says, “not used in writing of high solemnity.”
But the phrase, apparently only decades old, is very widely used. Blind condemnation sometimes takes an understandable statement and exchanges it for something confusing.
“I worked for free” is more clear than “I worked free” or “I worked for nothing.” “It is impossible to live for free” is not the same as “it is impossible to live free.”
If the meaning doesn’t change, “free” is the better choice. But “for free” is too established and too useful to be disallowed.