I tihnk the "surprise" comes in because when we see a Result List we expect it to have the same functionality of any result list (for which we have permissions). The result list for a multi-cabinet search looks a lot like a normal list and appears to have a direct line to the documents we see. This is why it is odd when we cannot edit indexes.
I understand the complexity involved with editing multiple indexes -- permissions for one. What if someone wants to change multiple indexes across file cabinets but they actually only have permissions to change them for one of the cabinets accessed? That could get tricky.
But why not the ability to edit indexes on a single document? The result list knows the cabinet whence the document came, and knows the document ID. It doesn't seem like it would be that hard to allow the modification of indexes.
As far as no one asking for this, I can only assume that is because people don't use multi-cab searches very much. And this sort of thing could very well be the reason why. If such search results do not offer the same functionality as single-cab searches, of course folks aren't going to use it much. It feels odd that someone would even have to ask. When I come across an operation missing expected functionality (and there is no clear reason why it wouldn't be offered), I don't think "feature request" -- I usually think "bug", or at least "oversight".