"There is no reason why 'It’s free' should work better than 'Sign up for free'"
Actually I disagree. The phrase "sign up for free" contains some hint of a potential "but", similar to "free to join" or "download trial for free", or other carefully worded phrases that litter anything-but-free internet offers.
Having said that, a hint of a potential "but" (i.e. trial-ware) is good at times so that the users don't take the free part for granted and they are aware of an upgrade option.
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"There is no reason why 'It’s free' should work better than 'Sign up for free'"
Actually I disagree. The phrase "sign up for free" contains some hint of a potential "but", similar to "free to join" or "download trial for free", or other carefully worded phrases that litter anything-but-free internet offers.
"it's free" is far more emphatic and open.
Maybe. But "Free Sign Up" sounds/works just fine.
Having said that, a hint of a potential "but" (i.e. trial-ware) is good at times so that the users don't take the free part for granted and they are aware of an upgrade option.