The extra guest “fee” would more accurately be called the nightly rate for extra guests.
Service fees are administrative fees.
And I think it is important for guests to understand what goes to the host and what goes to Airbnb. Sure, the bottom line is what the guest pays in total, but guests who understand that hosts do not see a penny of that service fee are more likely to give higher value ratings.
Because if they understand that the host didn’t pocket that service fee they can better understand why the host charges what they do for the nightly rate.