Understandable, that sounds like a major pain. Hopefully the anti-spam does not impact reputable anonymous users, but I can only imagine the trouble caused by the influx of spam.

Good question. It's not quite the same.

The most compelling reason is that browsing an onion service does not leak any information about the destination to an exit relay because the connection goes directly to the destination service. Connecting via an onion service makes timing correlation attacks much harder to carry out to deanonymize users since there is no exit relay to record when connections to lemmy.dbzer0.com are made. Posts and the timestamps associated with them on a public social network make timing correlation attacks even easier to perform, since there is evidence on which to validate the results.

It also acts as an advertisement about the site's commitment to anonymity and privacy.