I started to get into doing the whole chase tour thing several years ago and the insurance issue was one of the reasons why I quickly quit doing it. As others have mentioned the specifics are going to vary by state and how exactly you setup your tours and business. I wouldn't mess with it unless you have a lot of money to invest and are really serious about it. Personally the biggest reason I didn't want to continue doing it was because of the time requirements and the stress involved. Forecasting and getting your own group of chase buddies organized and out of town is hard enough during chase season. When you start having to work with customers it gets exponentially worse. I felt a lot more pressure to spend time on forecasting and looking over data in the field. It kind of took the fun out of chasing for me. It also didn't leave me time to work on other aspects of chasing like posting on my blog, shooting video in the field and reporting back to the station when I'm in Kansas. And as weird as it sounds, I also think doing tours was detrimental to how effective I was at getting us in the right spot. I say that because you start allowing things other than forecasting determine where you target. Before I started taking people out chasing with me I would always target the area where I thought the best tornado potential was. I don't want to ramble on about the details, but the more you turn chasing into a business the more outside influences affect how you chase and how much you enjoy it while you do. I didn't get into the chase tours because I wanted the money, so it was an easy decision for me to get out of it. I honestly just wanted to do chase tours because I enjoy taking people and showing them how incredible a good chase can be. It just wasn't worth all the added pressure, stress and time required to set something like that up properly. The insurance thing is just one of many hurdles. Exactly what all insurance you need to do chase tours is debatable too. There are people out there doing chase tours with a rental car and no insurance and then there are people that do it with vans that are licensed and insured for what they are actually doing with them. So like I said, exactly what insurance is needed or required to do chase tours is a pretty gray area because there is a wide array of "chase tours" out there. I didn't like the idea of putting my ass on the line so I was pursuing the well insurred route with it. Getting insurance for something like chase tours is difficult to say the least and if you do find coverage then the costs involved make it very difficult to be profitable. You have to be damn serious about it and draw in a large number of customers if you want to be successful doing it with good insurance coverage because you can't disolve those high costs into a hand full of customers.
Hopefully that helps a bit. And for what its worth, personally I am a lot happier just doing my regular thing chasing. Doing the chase tours simply took away from all the other aspects of storm chasing I'm involved with and it wasn't worth it.