As an illustration why, consider this quote dug up by the First Amendment Center's David L. Hudson, who found it in a government brief signed by Kagan in United States v Stevens: “Whether a given category of speech enjoys First Amendment protection depends upon a categorical balancing of the value of the speech against its societal costs.”
The case concerned a statute that made it criminally unlawful to depict animal cruelty. The Court rejected Kagan's reasoning, but had the justices accepted her assertion, it would have effectively repealed the First Amendment's protection of speech and replaced it by granting government the authority to decide what speech should be permitted.
You can read the entirety of Kagan's brief here, and additional analysis by Hudson of Kagan's record on First Amendment issues here.
Read more at the Washington Examiner: http://www.washingtonexaminer.com/opinion/blogs/beltway-confidential/Kagan-Speech-is-free-if-government-decides-it-has--93309159.html#ixzz0nkwf1Cfw