I feel like there's a better answer here: "Because it's hard to un-shoot them later if we decide it was a bad idea." When uncertain what to do, keep your options open.
Superhappies don't act symmetrically with just anyone. Only with other symmetrists. If humans decided not to shoot them for the reason you gave, and the Superhappies found out, the human's interests would have been ignored.
Previous Open Thread: http://lesswrong.com/r/discussion/lw/k94/open_thread_may_19_25_2014/
You know the drill - If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
Notes for future OT posters:
1. Please add the 'open_thread' tag.
2. Check if there is an active Open Thread before posting a new one.
3. Open Threads should start on Monday, and end on Sunday.
4. Open Threads should be posted in Discussion, and not Main.