This is a non-sequitur. It's an "American" organization whose sole purpose is to lobby the US government on behalf of the interests of a foreign government. It should register as a foreign agent as should all its employees because that's exactly how they behave and why they exist.

I agree that AIPAC may qualify as a foreign agent. But in that case, there are ~hundreds of domestic foreign lobby nonprofits in the US that are effectively unregistered foreign agents; just about every ethnic group in the US has one.

(More generally, I think the idea of a "foreign agent" that isn't actively and clearly sabotaging a country is a jingoistic boogeyman. But we should avoid the appearance of special pleading if we're going to call AIPAC one.)

There's a name for that: Fifth column.