Its for two reasons as i understand it:
1: Stock is cheaper
2: So the club/regional drivers can race in a class that they can win or do well in.
I dont believe (in off-road) that its to create a level playing field. As pointed out earlier any decent branded equipment these days is pretty much similar, its very hard to get a performance advantage from your electrical system over another, you just need to know how to set it up right
