It's a chilling effect because the current view in the US is that something as seemingly innocuous as peaceably assembling (virtually in this case) and exercising your right to free speech would possibly result in more data being added to "your permanent record" which, it is believed, has the potential to be used against you in unpredictable ways.

To be more specific, this (let's say) metaphorical file could have entries in it as a result of you signing a petition to pardon Snowden. While such entries could be myriad and varied in detail, I'll pick two hypothetical variations.

1) "Subject exercised right to free speech online."

2) At 1820Z, Subject did willfully and intentionally announce their position against the protection of national security, the lives of US Armed Forces soldiers, the lives of US Intelligence operatives, and the lawmaking authority of the legislative branch of the US Government by supporting an individual charged with espionage. Furthermore, by doing so, Subject did willfully and intentionally foment rebellion within the US general population by the "social proof" of his anti-authoritarian position.

Not knowing if, or which, entry might be made by the mass surveillance of even public forums, where there is no expectation of privacy, is the concern. Not that the speech is public, but that the speech isn't expected to be used in unknowable ways against the speaker.