With the latest CSB kits, there is no sprue, and almost no flash or similar irregularities. When there are fewer irregularities to remove, it is less likely that a child will accidentally reshape a wheel. The car must fit our track. Most cars shouldn't have any problem with this, but it's good to include the relevant dimensions for those who want to build something more unusual. You can determine these dimensions for your race by measuring your track, finish gate, etc.
Some groups explicitly list a large number of techniques that are illegal in their derby.
The goal is to prevent anyone from achieving an unfair advantage, and any technique that is perceived to give such an advantage is prohibited. For example, some groups prohibit changing the wheel locations, or increasing the overall length of the car. Others restrict the choice of lubricants to one or two approved types. Ultimately, it comes down to what different people consider to be an unfair advantage. In general, I think it is better to make such advantages available to everyone whenever possible, rather than prohibiting them.