Why isn't the string returned by symbol-name read-only?

[Daniel Colascione]

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Consider

(let ((name (symbol-name 'foo)))
  (aset name 0 ?x)
  (cons name (symbol-name 'foo)))

That yields ("xoo" . " xoo")

Now try


(let ((name (symbol-name 'car)))
  (aset name 0 ?x)
  (cons name (symbol-name 'car)))

That returns in an error when you try to evaluate it. The object
returned by (symbol-name 'car) must have some read-only bit set.

Why isn't that bit set for *all* symbol names?
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