Fridge Logic: People see a lot of positive things written about a pig in a spider web and are thus impressed with the pig. No one seems to care that there is a literate spider.
Mrs. Zuckerman points this out, stating that they don't have a remarkable pig, they have a remarkable spider. Mr. Zuckerman dismisses it. The unspoken conclusion is that God put the words in the web, not some ordinary gray spider.
So, the Zuckermans see absolutely no problem in killing the pig that their niece spent so much time raising so lovingly for his meat? Does no one in this family, aside from Fern herself, see anything wrong with this at all?
Fridge Brilliance: Having Uncle win the blue ribbon and Wilbur win the medal isn't just a way to tease the reader's emotions, but also a way to ensure that neither of the named pig characters has to become pork chops. Wilbur survives because he's become a celebrity, and Uncle survives because, as the local prize pig, he'll be in demand as a stud boar.
There's also a very logical reason for Uncle to have won: his size. Wilbur, having been a runt, was lucky to have even reached average size. It was likely Fern's care that let him grow even that large.