It's rather naive apart from being ethically objectionable to assume that our investigators travel around the country with bags of money trying to bribe witnesses to lie on the witness stand. We just don't operate that way.
The state's interest in marriage is stability. Generally speaking polygamy does not work for stability. Inherent in the whole polygamous movement is a deep and abiding misogyny and denigration of women. So polygamy is objectionable on lots of grounds.
The car is a character in the piece - I've never liked the car I submitted to it's objectionable popularity.