It is likely the horses were valuable as we dont spend money transporting £300 nags that way. Although a US or Canadian landing was possible for tech issues, it may well have resulted in the horses being impounded or put down due to lack of paperwork. Say they were polo ponies and the result was the total loss of a team - the financial loss would be in the millions. Unfortunate outcome for the passengers but no real choice. Of course, all total speculation