Bosses are the hardest part of the raid, so why should you get 0 rewards for beating the boss for the instance if you didn't clear with them?
Considering how difficult these bosses are (often requiring VC), it shouldn't be a surprise that an instance might decide to warp someone in the help them with the boss (such as a player needing to leave). Imagine if you got warped in to help with Hycenea, managed to beat it against all odds and then get literally nothing LMFAO