Why doesn't the WTBT$ just tell their members that there's a $10 fee to attend?
It would be the same thing alright, but Watchtower has always made a big deal of the 'Admission is Free, No Collections taken' type of promotion. I believe this goes back to C.T. Russell's time and continues to today. (Below is a typical invitation - from N.H. Knorrs's era.)
So to get money they have to get sneaky about it and charge a per-publisher fee that average publishers have no idea that they are paying.