Spam Arrest is a spam filtering company which supplies a service to subscribers promising to cut down drastically on spam. It works in the following way:
The system requires the user to enter a pattern of letters (a CAPTCHA) to release an email. That sounds reasonable, but this release mechanism means that the user has automatically agreed to their licence agreement, which is in very small print and spaced so you have to scroll down to see it.
When the user agrees, they are also agreeing to idemnify the company to the tune of $2000 for every single email they allow which they (not the user) deem to be spam. The user doesn't waive any right of appeal, but they are agreeing it has to be done under US legal jurisdiction, with all that implies in terms of US legal costs etc.
This might be an unenforceable contract in the UK, but should anyone be caught by it, they'd not be able to travel to anywhere under US jurisdiction if this company decided (unilaterally) that spam had been sent unless they were in a position to appeal any judgement.
This particular company is called Spam Arrest but there are others that work similarly.
Some moderators of groups have decided to ban members using this and other spam filering companies as they consider it an unnecessary risk to their members who might reply to messages.