The reason he decries action for publicity's sake is largely because it shows a lack of sincerity - being pious solely for personal gain is the exact opposite of sincere devotion. Contrast this with evangelism and preaching, the entire point of which is to communicate a message publicly - Christians are commanded to go out and preach the word of God. I'm pretty certain the WBC are pretty sincere in their teachings (you'd have to be to endure what they do - no-one is that committed to being an IRL troll), and I'm pretty certain what they're doing are not inappropriate public displays of private devotion, but rather public evangelism, prophecy and preaching. They not only tell people that they're going to hell, they explain why they believe that and back this up with Bible verses. Then they tell them how they can repent and be saved. Sounds like a pretty typical evangelical MO to me (though the political stances are slightly more extreme than is typical).
EDIT: In fact, on the topic of sincerity, I'm willing to bet that the WBC are a damn sight more
sincere than most Christians when it comes to actually preaching what the bible says. It doesn't exactly take a conservative view of the bible to end up with the GOD HATES FAGS message, for instance - it's just a liberal reformist movement within major churches coupled with the glacial inevitability of social progress that has started to change the message to one of tolerance. The WBC are just better at sticking to their guns.
