> It's not clear to me why regulators want to prevent mutually agreeable transactions from taking place.

In general, because the regulatory functions are aware of information asymmetries that could lead to large numbers of people transferring a significant amount of wealth to smaller number of other people.

Wealth concentration is expensive for society, because those who lose out do NOT give up their right to vote replacement money into their pockets. The more of them there are, the more likely they are to get society to cover some of their losses.