Not a much thought went into that decision, honestly.
The main reason I chose an identifier there was because "as" always stores a simple name, so allowing it to potentially be other contexts or expressions (which are illegal) just seemed to create complexity. I saw we used simple identifiers in many similar places and just went with it.
(As a related note, if mapping patterns had their own AST nodes, I would probably choose to represent "**rest" as an optional identifier, rather than a keyless value or optional Name.)
If it causes problems for clients of the AST, it's easy to change. But I sort of like it a bit better how it is now.