I consider it harassment of the people attending the funeral who are not being dicks.
Harassment is defined statutorily, and WBC's actions do not fit the definition.
The people who organized the funeral have rights to the land for the duration of the ceremony,
And WBC is not on that land. They are on public property near the land, but not on the land itself. Also, funeral goers do not have an exclusive right to the land during the funeral. Other people can (and sometimes will) come into the cemetary for their own purposes, even with a funeral going on. Most of those people will avoid the funeral, but they have a right to be there.
You have the right to free speech, but you don't have the right to intrude on a private ceremony or place and demand that people put up with you.
Actually, that is what the right to free speech is about. You have every right to speak your mind (so long as you do not incite violence, make viable threats, or spew obscenities), so long as it is done in the proper time-place-manner. WBC complies with the time-place-manner restrictions when they do their protests, and they themselves do not incite violence. (Bystanders want to hurt them, but that is not the legal definition of inciting violence.)
If I ran a restaurant and some LaRouches came in and started interrupting meals and the general peace to spread their horsecrap, then I should have the right to refuse service to them and to throw them out.
Your resteraunt would be private property, whereas WBC protests on public property. Also, you could throw them out if they were doing something that meets the legal definition of "distrubing the peace" (which varies slightly from state to state), but if they were not on your property, there isn't much you could do.
There is a difference between being offended and being harassed during an emotionally vulnerable time.
But there is no right not to be harassed (other than general harassment laws, which WBC obviously does not break, else they would have already been arrested for it).
SCOTUS has generally upheld time-place-manner restrictions though (well, many of them anyway). It's been a while since I looked at the case law on this, but there is a possibility that it could withstand a constitutional challenge.
Time-place-manner restrictions mean there must be a viable time, place, and manner in which a protest can be carried out, the restriction must be content neutral, and it must be narrowly tailored to serve an important government interest. They are valid, if a protest could reasonably be conducted within them. For example, saying "no audible protests within 50 feet of the courthouse while court is in session" would be valid, since you could (1) protest audibly 51 feet away, or (2) protest silently within 50 feet. Saying "protests are allowed between 1AM and 4AM" would be too restrictive because the limitation isn't narrowly tailored.
I'm on the fence, but I can, actually, see how this is not an infringement on their freedom of speech. If I'm reading the article correctly, then if they really want to protest at a cemetery, they still can do that, just not during a military funeral or for two hours before and after services. They can still protest there, just not during those times.
An argument can be made that it isn't infringing on their freedom, but it is setting a time and place limit that basically robs them of their target audience.
It would be an interesting legal argument, and to be honest, I'm not sure how it would come out. On one hand, it does meet the time-place-manner limitation. As you said, they can protest whenever and wherever they want, so long as it isn't 2 hours before or after a funeral or within 300 feet of the mourners. The law is clearly content-neutral; just because it was written to address WBC does not mean only WBC would be affected. (Anyone who protested a military funeral for any reason within 2 hours or within 300 feet would be subject to the law.) But I am not sure about the narrow tailoring. Is this the least restrictive means available?