Why is the pure tungsten (W) melting point higher than that of its oxides (either WO3 or WO2)?
As far as I know,

$\ce{WO2}$ and $\ce{WO3}$ have a covalent characteristic, and thus lower melting point than the metal.

while, $\ce{Al2O3}$ have more ionic characteristic, and thus higher melting point than the metal.

$\ce{W}$ is a d-block metal from the $\ce{3^{rd}}$ row which make it possible to have covalent materials rather than ionic materials.

this article might prove this point 
http://pubs.acs.org/doi/abs/10.1021/jp963724z?journalCode=jpcbfk