Because Framework is not inside BRL.System, and therefore a second Framework would not be recognized? If that's the case, that's silly, because Framework is a compiler directive, is it not?
No. Take this example :
Framework BRL.System Framework BRL.Blitz
The first line says "Import no modules except BRL.Mod1"
The second line says "Import no modules except Brl.Mod2"
So in other words, the second line, removes the module you just imported with the first line. It's counter-intuitive to allow this. It's a compile-time directive, so you can't change your mind in the code. There's no purpose to doing it twice.
It's nothing to do with the modules which contain Framework, as it's not in a module, as you say it's a compiler directive.
Intuitively speaking, it would make sense to add allowed modules to subsequent Frameworks
Not really, because Framework is for removing modules, not adding them. By default, they're all added. Framework is saying, don't import them all, I only need this one.
or allow multiple module includes on one line:
Teah, that'd be fine. It's only alternative syntax to a feature we already have, but it would be perfectly intuitive and wouldn't break anything. I'd have no problem at all with that alternative/additional syntax.