The First Amendment protection of speech is guaranteed right to all citizens.

The picketing was done peacefully with no shouting or violence. The authorities were notified ahead of time and monitored the protest.
The picketing took place 1,000 feet away from the funeral service, out of sight from the church the service was being held at.

Westboro’s speech was at a public place and on a topic of public concern. The speech is entitled to “special protection” under the First Amendment.