Dunno about the XTZ660 but the XT600Z does just that. If there's (a) too much oil or (b) the pressure is too high, for example due to high oil temperature, it all ends up in the air filter box. I actually have a rather thick black hose on the right side coming from the engine (methinks... either that or the oil tank) straight into the air filter box. Assuming the XTZ660 does the same thing it is technically possible that they put in too much oil, which again is a rather frequent mistake.
"Life is not a journey to the grave with the intention of arriving safely in one pretty and well preserved piece, but to skid across the line broadside, thoroughly used up, worn out, leaking oil, shouting GERONIMO!"