He is very very rich, if he had particularly cared about protecting the fields rather than shooting elephants perhaps he would have paid for the installation and upkeep of a fence.

His own comment on the affair was "These people have literally nothing and when an elephant is killed it's a big event for them, they are going to be able to eat some protein." Now personally I supect that there are better ways for Bob to go and increase the protein in these people's diets, other than shooting elephants.