It doesn't matter if you disagree with them, it is a free speech issue.
They can picket all they want, just as people are free to picket abortion clinics and religious institutions.
Free speech is protected on public land people. They could be saying absolutely horrible things, but as long as it is not a) incitement to riot, b) fighting words, or c) hate speech, it's constitutionally protected, and if we decide to ban it, it won't be long before dissent against the government and its policies are restricted too - and that's grounds for a revolution, according to our founding fathers.
EDIT:
historybuff said:
Man, I think if some dickheads like that were going to use their freedom of speech to picket at my dead son/daughter's funeral...I'd just have to exercise my right to go kick their asses.
You don't have that right. Period. You'd be locked up, and quite right too. Now, I'm not saying I wouldn't sympathize with you, but you still can't do that. Find a better, non-violent, legal way.