But the reason is not greed. You may not be aware that a personal account accidentally left logged in is very dangerous to the account holder. Someone can come along, change the username and password, and they've taken over your account!
Further, there may be an unknown security flaw that could allow the unscrupulous to order hard goods with your credit card and have them sent somewhere else. Theoretically, Ancestry has gone through several security audits and this is not possible. But believe me, they take the threat of this very seriously.
Word in the halls is that Ancestry is searching for a safe and officially sanctioned way of accessing non-FHL accounts at the FHL.