Buffy the vampire slayer and Angel unavailable

feline1 

I am only seeing Prime's offerings as rent/buy only for both Buffy and Angel.

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This is kind of the thing with streaming.  Content is shared and licensed between all the different services for definite periods of time.  All sorts of tracking software is implemented to see the popularity of a show, what the cost is to renew a license, whether the show is currently available to be licensed, etc.  

The decision to carry, or not carry, a particular show is not always up to the carrier/licensee.  The distributor and licensors play a role also.  The streaming services like Roku and others have to do a balancing in order to keep the library refreshed and, most likely, within a certain budgetary number.  Can't just purchase licenses and keep content indefinitely like a storage server.

When something a user watched disappeared off cable, did the user call the cable company?  No. Basically stuck with watching what was on the other channels.  With streaming, if you remain flexible and look for your desired content elsewhere (ie. other channels/apps), you are going to find a very big streaming world out there and may find other channel/apps that you really like and can add to your alternatives. 

I rarely subscribe to the paid subscription services like Hulu and the like, but would consider a free offer here and there if there was something I really wanted to watch.  Just note, you can usually only take advantage of a free offer for a particular service once, unless you use different email addresses/different Roku accounts. 

For what its worth, I have seen Buffy offered before on the Roku Channel at different times, so it will likely return in the near future.