I think it is a more plausible slogan than the usual version. If you and I disagree because I want an outcome more favorable to me and you want an outcome more favorable to you, there is room for compromise—as we see whenever people bargain over the price of a house. But if we disagree because I see what I want as just and the alternative as unjust and you see it the other way around, compromise looks to both of us like moral treason.
Consider the issue, currently a live one in Europe, of whether people should be fined for saying or writing things critical of Islam. For those who support the traditional liberal view, agreeing to a fine of five hundred dollars instead of a thousand dollars isn't a solution—any punishment at all is an intolerable violation of free speech. For some orthodox Muslims, on the other hand, permitting people to slander the Prophet is clearly unacceptable; if the government will not impose a fine large enough to stop such an outrage, it is up to the believers to stop it themselves.
That, I think, is part of the nature of beliefs about justice—they are absolute, bright edged, in a way in which preferences are not. The point is summed up in the Latin phrase Fiat justicia, ruat coelum—let justice be done though the sky falls.
Those whose bumper stickers read "If you want peace, work for justice" simply take it for granted that there is no question what is just; if you want to find out, just ask them. The problem with the world as they see it is merely that other people are insufficiently virtuous to act accordingly.