Fortnite is changing what skins can be equipped and where, including real-money paid for skins. Specifically, labeling some skins as not E™ for Everyone™ and only allowing them in non-E™ zones.
People are upset that the cosmetics they paid real human dollars for are going to be unusable where they want to play, defeating the entire point of spending money on a cosmetic.
People are also upset at seemingly arbitrary lines-in-the-sand drawn over which skins dont count as E™, and which do. In a “why is my skin off limits but that skin which is far less kid friendly is no issue?” style complaint.