Common sense rarely seems to apply in court.
While there are limits to free speech, most notably like those you mention where people can be hurt, maimed or killed as a result, the crap that comes out of Phelps' mouth isn't the same thing. IMHO, it's quite within his rights to say mean spiteful things about dead people. They are certainly quite beyond caring.
Certainly, Phelps is an bellybutton for doing so. Certainly, we can all hope that Phelps burns in some fiery hell, but I doubt very much that there is any legal ground from which the ACLU can fight to stop Phelps from exercising his right of free speech no matter how distasteful it may be. IANAL, but I disagree that 9th and 10th Amendments apply.
"I do not agree with what you have to say, but I'll defend to the death your right to say it." - Voltaire