Because post previews show up in the Active Threads and Active Posts feeds, so if folk use them (as they should be able to) it's very easy to see spoilers accidentally if they're not in tags.
Thanks for the explanation, but it seems like a pretty cumbersome system.
Wouldn't it be more convenient if it was possible to mark the entire topic, instead of individual posts?
Or at least the "UNHIDE ALL SPOILERS" button (reading would then be much more comfortable).