They operate on a block policy, i.e. covers any person for any risk whilst using their vehicles - rather than insuring the one person. If you hire out 20,000 cars a year for the whole year, and 200,000 people drive them, the risk is spread over the 200,000 people. Out of that, only 1000 may be American, you have a one in 200 x 365 days x 20,000 = 1.4609 million to one chance of claim (in principle). Compare this to an extremely high probability in comparison of one American driving in the UK on your policy, or their own policy on your car. There's no one to split the risk with. The probability of a foreigner with no knowledge of our highway code, road system, driving a right hand drive car on the left hand side of the road for the first time creates a high probability of risk.