The 'blackmail' in this case is 'We will keep all of A2B on the SBL unless you disconnect one of your paying customers from the Internet entirely'
They don't care that A2B doesn't send SPAM they just really dislike the cyberbunker and are abusing the power of their SBL to try and remove entities that they don't like from the Internet entirely.
Note that the cyberbunker will remove spammers, their ToS:
This includes but is not limited to:
* Any criminal activities, as defined by the law of:
o Customer's country of residence (natural persons) or company registration,
* Mass sending of unsollicted e-mail (SPAM).
* Sending floods of any kind to any other computer system or network which inhibits the correct behaviour of said computer system or network.
* Harrassment of individuals.
* Endangering the quality of service or network stability for other internet users in any way.
It's just that if spamhaus says that someone is doing 'something illegal' without specifying what it is, the cyberbunker isn't just going to disconnect them. Spamhaus *really* dislikes it when an ISP doesn't blindly follow their 'opinions' and most ISPs do because if they don't they get blackmailed like Cyberbunker was before and A2B is now.