The bug
After placing an enderportal frame in creative mode and then inserting all eyes of ender the portal appears. If a part of the frame is removed or destroyed the enderportal still remains and the player can still travel through it. Even when all the frame pieces are removed the portal still doesn't disappear. It is even possible to remove parts of the portal and then travel through it.
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It is possible to place an end portal block by itself (/give 119 or something), so I don't think this is a bug.
Most likely, as Tulir Asokan mentioned, it is normally not possible to remove pieces from the portal frame, so no checks were coded to detect such removals (they'd be mostly useless).
For comparison, Nether portals (whose frames can be messed with in survival) have somewhat elaborate piece of code to detect problems in their frame or portal blocks, and then shut down the rest of the portal blocks when necessary.
So yeah, not a bug, probably not an intended "feature", but just left unimplemented as it is not really an issue. (Edit: not an issue, since you can remove all the portal blocks in creative.)
@Anon Ymus: It is also possible to give yourself a Nether Portal block, but they are destroyed when you update them and also they are destroyed if you destroy the portal frame.
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