How is picketing a funeral any different than picketing any other day of the week? A wedding? A birthday? Someone just having a crappy day who doesn't want to be bothered? State of mind is a weak argument -- on those grounds, I could tell someone my dog just got hit by a car and I don't want to see anyone trying to picket my gay rights event because it would just further upset me...or maybe I just had a baby, and I just don't feel like dealing with anyone's shit trying to bring me down, because who wants to raise their kid in a world that's mean and spiteful?
They're not even picketing AT the funeral or even on the funeral grounds. Nobody is putting a top hat on and tap-dancing on anybody's grave. In fact, WBC usually shows up for a very negligible amount of time and then leaves. They don't know you, they probably don't even care who you are or what your name is, they just show up and then leave when their time is up and make absolutely no lasting impact except a wave of unnecessary outrage.
Venue choice can affect freedom of speech and assembly. What good is protesting if the only place you're allowed is 10 miles out of town? Why does anyone care what you're saying if they can't see or hear you? Venue choice can also affect the strength of the message you're giving. For WBC's purposes, no matter how much you disagree with them, the venue choice strengthens their voice and makes people take notice. For all intents and purposes, they are highly successful at being activists for their cause.
This is hardly inciting a riot -- the only violence being brought up is by everyone else BUT the WBC. Their words aren't directly stirring a panic the way yelling "fire" would, they are NOT inciting direct violence by saying things like "go out and beat up a homo today" -- in fact, they are peaceful, as much as that word goes against their message. They smile, they take the abuse others give them, and they practice their legal rights while obeying the law, which, now that I've examined both sides, is actually a lot more than I can say of those opposing them. Being on the perceived right side of an issue is not a carte blanche to do whatever you want without regards to the opposition. You can still be right in your stance on an issue and make the wrong choices.
They hardly deserve sympathy -- their opinion certainly doesn't merit much respect and nobody has to listen to them fairly if they don't want to, but disagreeing with someone's view, however appalling you find it, is never grounds for taking away their right to express it. Ever. I stand by their rights and I would defend to death their right to say it, as the saying goes. In fact, I think it would be hilarious if they picketed my funeral. I wouldn't want my family to give them the time of day or the satisfaction of being upset. I would want them to laugh at how much time they're wasting and encourage others to do the same.