Yes, indeed WBC is an abomination. But they aren't going to go down quietly. Threats and violence begat threats and violence.
If WBC didn't write that letter, that anonymous group.....abeit maybe with good intentions, (some anoms do seem to thrive on publicity too) is threatening to break the law, which some anom groups have done before.
I'm positive that the WBC didn't write the letter... anons have been waging a verbal & blogging war against the WBC for years. After I videotaped their protest in Los Angeles last year & spoke with Shirley Phelps-Roper, I was contacted by her son, Nate, who saw my YouTube video and we had an interesting chat about the origins of the Phelps family and why they're doing what they're doing. He said Shirley and her daughter, Megan, are the ringleaders of the protests and they get a "narcissistic buzz" from all the knee-jerk reactions to their protests.
When the reactions to their protesting the gay & AIDS community died down, they decided to up the ante & start attacking American soldiers & protesting at their funerals. Nate said it was wrong to presume that everyone hated the WBC and that if the IRS audited his sister's finances, they might be in for some surprises, as she receives thousands of dollars in donations from supporters that help finance their protests across the U.S.
I emailed him & asked him about the letter and he replied that his sister would not have written the letter, but it was certainly the means that gave his sister the impetus to react. I believe some anon wrote it knowing the reaction it would get from Shirley & Megan.