As all plantable crops in Minecraft get sped up by bees flying over them with nectar on their butts I think hemp should do so too. Please let me know if this is somehow intentional or just an oversight to be fixed :)

Bees are hardcoded to work on instances of CropsBlock
IE's hemp isn't one of those, nor does it grow like normal crops. IE's hemp grows to a heigh of two and uses custom logic to accomplish that.

Hence, bees can't work on it, because there is no hooks in Forge to make that happen.