From what I understand, if a TV channel (ABC, CBS, TBS, Fox, CNN, Showtime, HBO, etc.) shows a program that was originally ever aired on its network and is shown on the channel's website (ABC.com, CBS.com, TBS.com, etc.) then it MUST be captioned (within certain timetables, etc.).
This means that if NBC puts "Saturday Night Live" episodes on NBC.com they MUST be captioned. BUT if NBC sells the show to Hulu.com or Netflix.com the law DOES NOT REQUIRE these "third-party" sites to add captions. Also, if NBC makes webisodes on NBC.com that are never shown on regular TV, the law DOES NOT REQUIRE them to be captioned.
So Netflix, Hulu, Blockbuster, Amazon, etc. are all "exempt" from the law because they are not under a Broadcast network. FCC can only regulate broadcast networks.
HOWEVER, I heard the US Department of Justice is looking into seeing how the ADA might be applied to Netflix, Hulu, etc.