yes, it is mandatory, for parties of 6 or more. you cannot legally leave without either 1. paying the 18% or 2. speaking to a manager about having it removed because of poor service. you will find this exact same thing on the "tipping info" thread on the resatuarant board.
the reason Disney requires this (as do many restaurants) for parties of 6 or more is becasue larger parties take up a lot of the servers time . disney requires this, even at buffets (for parties of 6 or more.)
just correcting the misinformation, so people aren't surprised.
of course,if you recieved bad service, speak to the manager about having the tip removed.
ps. a party sitting at the same table is one party, even if you are paying separately. 2 families of 4 each, at the same table, is a party of 8, no matter how many checks are required.
If you choose to try to sit at separate tables in order to avoid the automatic tip, you are not guaranteed to be next to each other,or even seated at the same time.(if you choose to do this, please don't "talk over" the poor people sitting between you)