Thanks for bringing this up. This behaviour is currently by design. I'm not sure that we want to make this change, just because people might find spoilers in search by accident (which defeats the purpose really). If the community agrees en masse, then we'll consider the change.
Another option is to make the text in the spoiler searchable, but not display spoilers in the search results. This is definitely possible (if the community wants it).
For what it's worth, I can confirm that we do not index spoilers currently. This is the entire body of the search document that represents the post you cited:
"body":"Spoilers for Saw 3D ahead:\n\n\n"
You can see that the spoiler was not included.