A recent ruling by the U.S. Ninth Circuit Court of Appeals could technically have legal implications for those who share Netflix and HBO Go passwords (so basically everyone). The case did not deal with Netflix or HBO specifically, but it did find that sharing passwords is partially a crime prosecutable under the Computer Fraud and Abuse Act (CFAA). This decision dealt with an employee of a headhungting/recruiting firm who used a password to access company data after he left.

Sources: A.V. Club | U.S. Courts