Because this is text box is displayed through a javascript library called CKEditor, and the default configuration option that disables your native browser spell checker is in place.
This would be a question for Patrick and Alix. (For more documentation on the one specific setting: http://docs.ckeditor.com/#!/guide/dev_spellcheck)
Viva la dictionary book
(but hide it from the ipolice)
(image stolen strike that borrowed from here http://philologophile.blogspot.com/2010/09/my-1838-websters-dictionary.html (thank you))
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