If you purchase a licensed cake topper you have the right to use it as you see fit (including selling it) as long as you don't copy it, this right is protected by the doctrine of first sale.
Since people buy Wilton products to prepare these cakes And Wilton sells cake supplies not cakes i would think they would be glad.
Wilton's goal is to sell more of its products and fill its classes. Encouraging people to follow copyright law would run counter to this goal, which may be why you hear stories about Wilton instructors neglecting to mention copyright laws or food safety laws (willingly or not).
I went to a baby shower and the person bought a cake from someone and it was Cat In The Hat no mistaking that is copyrighted.
Many people are either unaware of how copyright law works or just don't care...that is until they receive a cease and desist order or are sued.