We prove that for the cases $X=\mathcal{C}(K)$ ($K$infinite) and$X$=$L$_{1}(μ) (μ σ-finite and atomless) it holds that every slice of the unit ball of the$N$-fold symmetric tensor product of$X$has diameter two. In fact, we prove more general results for weak neighborhoods relative to the unit ball. As a consequence, we deduce that the spaces of$N$-homogeneous polynomials on those classical Banach spaces have no points of Fréchet differentiability.