I don't know why. If I had to guess, I'd say perhaps it's because firefighters and police officers have more reasonable unions and/or compensation packages, but that's pure speculation on my part.
Personally, I think public sector unions should be banned. It was a nice experiment, but it failed. Here's the problem with public sector unions. I think we can all agree it would be wrong, not to mention illegal, for a worker or a union to bribe a company's management into giving higher salaries and benefits. That screws over the company's stakeholders. But with the public sector unions, that's exactly what's happening, except it's legal. Politicians give unions what they want and unions reward politicians with campaign contributions and political support. They both win and the taxpayers get screwed.
I also think that joining a union should never be a condition for accepting a job. As it is right now, if I were to become a teacher in Wisconsin, I would have to join the union and pay dues. It doesn't matter if I despise everything the union stands for (and I do) or if I oppose the politicians the union supports with my money (which I probably would). I still have to pay. That's not right and that's what they're trying to change in Wisconsin.
I'm against public workers organizing. I don't know what my financial situation has to do with it. Yes, I do have some assets, but I wouldn't call myself rich (yet). What I have I acquired through non-union work. I have never been in a union and I've never felt I my compensation was unfair.