I see it as an attempt to multiply the spam effects, like throwing cash in the street to create a flash mob.

If many people see they can grab the spam, they will attempt to grab it, creating thousandths of dust collection transactions, which will further clog the network.

If the spammer was sending to a private address and collecting the dust, that would be just two tx.

By sending to a trivially redeemable output, he can expect many tx (1 for the dust creation, then many for the people attempting to grab the dust)