The gentle reader can read Foust's ruling here but in brief, he found that the law illegally forced unions to represent non-members without just compensation. To make it worse, this was mandated by the state for the "benefit of the public," even though doing so was in direct violation with state and federal labor laws.
In other words, per the standing law, if a union is representing a majority of workers at a place of employment, it is because the majority of workers voted for it. The law also forbids unions from discriminating against nonmembers when they represent them in contract negotiations, grievances and/or disciplinary hearings. In compensation for their services, nonmembers are required to pay a nominal "fair share."
The Right to Freeload law was just that. It said that unions had to provide these services without being compensated for it. In other words, it was legalized theft. It would be like a person taking a taxi but then refusing to pay the fare because they chose not to.
The really ironic part which the ruling didn't address is that the Republicans were authorizing the theft of the unions' services because it would benefit the public by creating a more favorable business climate for the state.
However, the proof is that it did anything but that. Wisconsin is still lagging behind the rest of the nation in economic growth and job creation. Of course, this is not surprising unless one was gullible enough to think that Walker and the other corporate puppets were actually being honest about their reasoning for it.