Extension methods usage - Isn't this bad design?

public static class CategoryRepositoryExtensions
{
    public static Category GetRootCategory(this IRepository<Category> categoryRepository)
    {
     return categoryRepository.GetById(1);
    }
}

ICategoryRepoitory : IRepository<Category>
{
     Category GetRootCategory();
}

using CategoryRepositoryExtensions;
...
Category c = r.GetRootCategory();

Category c = CategoryRepositoryExtensions.GetRootCategory(r);

<IEnumerable> orders1Enumerable = OrderRepository.GetAllOrders()
                                     .ContainingProductCode(x);

<IEnumerable> orders2Enumerable = OrderRepository.GetAllOrders()
                                     .ContainingProductCode(x)
                                     .WithShippingZipCode(y);