Witnesses .... are you enabling bullying on HIVE?

I guess technically yes, I enable bullying.
In the sense that I run software that gathers collections of transactions into blocks, some of which may be downvotes or comments. I don't currently, nor do I plan to censor any transactions on HIVE.

Is it against your rules to use Hive account@steembasicincome?

The first rule is that there are no rules. This is a decentralized network made up of individual stakeholders, each with their own personalities, likes and dislikes.

I don't have a "rule" against steembasicincome, but personally see steembasicincome as a ponzi scheme and have flagged it's accounts many times in the past (mostly when I saw automated reports at the top of PALnet trending).

I don't specifically seek out users of the service for negative curation, but I am biased against SBI after seeing years of low effort reward farming by users.

If not, why have you let this down vote bullying continue and why have these accounts not been added to a blacklist?

a) I don't recognise downvotes as bullying.
b) I don't run a blacklist service personally, and I'd never consider blacklisting someone because of their voting habits anyway.