It's always a matter of rights.
Can't tell for sure for Angry Birds on Roku, but probably the Angry Birds license Roku had has expired, and they didn't renew the license so the app was deleted from the network.
Same goes for any game; a developer will license a distributor to 'use' the program with their devices, or even as a marketing tool. The license has a beginning and an end, and both parties can terminate it. The same model applies to music and movies business.
If a game developer goes down, who owns the rights to the games they created? If the title doesn't get sold to another company, it is simple taken down.
sv
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