Exactly like NSFW, accounts that properly label themselves are no problem, it's when they remain unlabeled that they cause problems.

PieFed both allows an account to label itself and provides a suite of tools to help mods identify whether an account is likely to be a bot or not - e.g. someone who never comments or votes on anything, but manages to post multiple times per minute (or second?).

Yes it's possible to filter/block if it's contained to one instance or community, but mirroring all that is such a waste of resources.

Obv, I think they should be clearly labeled as such with their purpose and that they should stick to a specific instance / dedicated community (thus making it easier to filter out their content if you don't want it)

But if they're doing all that, I don't think I see what the issue is? Can someone explain their grievance on the matter?