After some quick googling, I found more references to this claim, but only in comments on articles:
http://www.networkworld.com/community/node/37608:
"The use of a corporate jet is often required by corporate policy intended to protect the executive from the threat of violence or abduction. "
http://www.swamppolitics.com/news/politics/blog/2008/11/auto...:
"Most corporate CEOs in companies with corporate jets are REQUIRED to use those jets as their ONLY mode of travel, as per the insurance requirements of being the Chief Executive Office and Chairman of a Publicly Traded Company.".
http://www.networkworld.com/community/node/37608: "The use of a corporate jet is often required by corporate policy intended to protect the executive from the threat of violence or abduction. "
http://www.swamppolitics.com/news/politics/blog/2008/11/auto...: "Most corporate CEOs in companies with corporate jets are REQUIRED to use those jets as their ONLY mode of travel, as per the insurance requirements of being the Chief Executive Office and Chairman of a Publicly Traded Company.".