Internet Service Providers that bargain their users privacy to avoid legal action came under fire Thursday from a coalition of civil liberties groups.

Members of the ACLU, The Center for Democracy and Technology, Electronic Privacy Information Center (EPIC) and the Electronic Frontier Foundation and others sent out letters to more than 100 ISPs asking them to "adopt policies protecting their users' right to anonymous speech on the Internet."

The complaint comes after years of what the privacy groups identify as "cyberSLAPP" lawsuits - where a company files a suit just to discover the identity of their online critics - often in order to silence or intimidate them.

In a cyberSLAPP suit, the target of anonymous online criticism typically files a lawsuit against a "John Doe" defendant and then issues an identity-seeking subpoena to an ISP.

The most common complaints by cyberSLAPP plaintiffs are defamation, trademark or copyright infringement, and breach of contract. Speech that involves a public figure - such as a corporation - is only defamatory if it is false and said with "actual malice." It also must be factual rather than an expression of opinion.

Currently, ISPs have no legal requirement to notify their customers before complying with such subpoenas - even though the privacy groups claim many of the lawsuits are frivolous and have no chance of succeeding in court.

"You can't fight to protect your privacy and anonymity when you don't even know that it's being attacked," said Paul Levy of the Public Citizen Litigation Group.

The letter campaign comes one day after some of the same civil liberties groups issued a study, a white paper and a verbal broadside to the cable industry claiming cable high-speed Internet systems are being built to "block content."

The alliance noted that four major online service providers - Yahoo(YHOO: Quote, Company Info), Earthlink, Microsoft (MSFT: Quote, Company Info), and America Online (AOL: Quote, Company Info) - already notify their customers when they receive subpoenas for identifying information, and urged hundreds of other webmasters and service providers to do the same.

"These companies have led the way," said Cindy Cohn, Legal Director of the Electronic Frontier Foundation. "But too many online companies and discussion hosts still aren't protecting their customers from these privacy-invading abuses of the legal system." Ironically, one of the better-known cyberSLAPP cases involves AOL. Two years ago, New Jersey Superior Court Judge Miriam Span ordered AOL to disclose the identities of cyberspace John Does after the Elizabeth Board of Education received as many as 60 identical messages to nearly 2,000 teachers and board employees.

As a legal stance, the groups pointed out that the Supreme Court has repeatedly found that anonymous speech is a right protected by the First Amendment.

In the 1995 case of McIntyre v. Ohio Elections Commission, the justices ruled 7-2 that "government may not constitutionally prohibit the distribution of campaign literature that does not contain the name and address of the person issuing it."

"Anonymous speech on the Internet lets people make criticisms that are difficult to state openly, and share information and support about topics that might be stigmatizing, such as addiction or sexual abuse," said Ann Beeson, Staff Counsel of the American Civil Liberties Union. "Unless online anonymity is protected, whistleblowers who want to criticize their employers, parents who want to criticize the principal of their children's school - and many others - may be afraid to speak out. That would be a loss for our country."