I agree that a cake cutting fee is unreasonable if the vendor does not offer cakes or desserts themselves. However if vendor does offer cakes and/or desserts I believe it is very reasonable to charge a nominal fee for preparing and later
washing dessert plates, forks and other utensils used to serve a cake that not only did not bring in any money for the vendor, but that caused the vendor to lose out on that cake/dessert sale.
Severs are paid for their time and service, but the owner of the venue is going to spend energy and resources to set up a nice table for the cake complete with tablecloths that match the color scheme of the banquet, plates and forks that have to
be washed before and after the cake service, extra napkins, and they will also have to clean up and dispose of the waste that is made after the cake has been cut. I work for a restaurant that provides cakes and desserts, so as a customer I
would understand that if I want to bring my own cake to a restaurant that provides their own cakes and desserts, I would be happy to pay for the convenience of not having to provide my own plates, forks, cake table, and not to have to clean up the
mess after enjoying my dessert and not paying for anything at the place I am dining!
It would be rude to go to a restaurant and bring your own food, it's sort of the same thing with a cake (as long as the restaurant does offer some sort of cake or dessert)!