the hobbit - Why could the warg riding orcs attack in daylight?

Those were Hunter Orcs:

The fast-moving and fanatical Hunter Orcs serve as the right hand of Azog, a relentless band who will stop at nought to slay his quarry.

 

Speculation:

One of the Hunter Orcs ,Yazneg, was killed by Azog after he failed to capture Thorin, so they were clearly under a lot of pressure. Compared to Azog's wrath the Hunter Orcs possibly considerd sunlight to be the lesser evil.

Or maybe this breed, like the Uruk-hai, can indeed withstand the sunlight better

(Because this is 'non-canon' there seems to be very limited information)