Why does polonium form acidic oxides?
All metals form basic oxides. 


That's true. I cannot remember metal, that does not form basic oxide


  Metallic oxides are basic in character.


Not necessary. Polonium is not the only one metal, forming acidic oxides. Mn, Cr and some others do it as well.

Oxide is considered acidic, when it forms hydroxide, that is acidic (well, it is simplification, but let's stick with it for a while). Hydroxide is acidic, when it can lose proton, giving stable anion. Acidity of hydroxide and metallicity of element formed it does not connect any way. Acidity depends of ability of remaining part of molecule to stabilize negative charge, which usually correlate with $\ce{O/OH}$ ratio and electronegativity of central atom. So, $\ce{H6TeO6}$ is quite weak acid and $\ce{H2CrO4}$ or $\ce{HMnO4}$ are quite quite strong acids.