So you think that freedom of speech makes sense at these funerals? Well why is my freedom of speech limited on a plane when I announce I have a bomb? Your freedom of speech is more limited than you care to think.
You're legally allowed to say it as far as I know, nobody can stop you, nor can you be persecuted for yelling that you have a bomb on a plane. Unless of course the airline itself has some sort of policy regarding this behavior and you accept a contractual agreement not to engage in it when you purchase your ticket, which I believe is likely the case.
You can also be held liable for the deaths you cause, and I'd be willing to bet you'd go away for a long time.
Free speech doesn't exempt you from the actions and consequences incited by your words, it just gives you the freedom to speak them in the first place. It's not a "Say whatever you want and get away with it" rule, it's a "Say what you want but keep in mind you might be better off with your mouth shut" kind of arrangement. Some people just aren't very good at it.
By saying that the WBC should be silenced, you are making a judgement that somebodies opinion and beliefs are not valid, and that they should be persecuted for those beliefs, based simply on the decision that you and many other people don't agree with them. You live in a free country where people are allowed to believe what they want to believe, no matter how unpopular, far-fetched or radical, so long as they don't break any laws by exercising these beliefs.