No where in the constitution is the guarantee that we won't be offended by someone else's speech. We all know that. And that applies to the Phelps clan just as much as it applies to our speech. Are they wrong? Definitely. Do they have the right to be loud, obnoxious, and wrong? Definitely. That said, in most places, there are now thousands of counter-protesters to block them from the grieving families, which is also the right of the people.
If we don't tolerate their speech, what will be the next level of speech we won't tolerate?