It's not really about "treating" sewage differently. It's about how to get rid of effluent (treated, but brown) water.
Easier to say that aerobic systems won't get water off property via the sprinkler system, harder to say with a leech field or pump system.
I think it stated innocently, and is now just an extra fee the county gets (and now wants) for managing paper work.
Think of it like converting your garage into livable (air conditioned space), never tell anyone until you're ready to sell your house because you'll be taxed on the additional AC'd square footage.