Why sed -i does not respect the w mode?

[Peng Yu]

Hi,

I turned off w of test.txt. But it still can be changed by sed -i.
Should sed -i respect the w mode? Thanks.

$ echo a >test.txt
$ chmod -w test.txt
$ sed -i 's/a/A/' test.txt
$ cat test.txt
A

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Regards,
Peng