Studio is blissfully unaware of what tags are valid underneath other tags. We just offer the full set to the user. It'd be great if we had something like a DTD so we knew which tags are valid underneath others.
There are other cases where it'd be great to get the input from Alloy rather than Studio hard-code: i.e. "artificial" tags (tags that aren't real types, but may behave like other tags/types - LeftNavButton is not a type, but acts like a Button)