One is that they don't use the infrastructure in the state. Shippers are either already taxed for their use if private or self funding government in the case of UPS.
The second is a more slippery taxation without representation which also gets into all sorts of semantic issues with who is really being taxed the in state residents or the out of state residents.
Really the tax infrastructure is in drastic need of modernization and streamlining. It may step on a lot of toes but I could see forcing online sales tax into a federal fixed rate with redistributed fixed percentages as worth it. Say 5% rate nationally and of that 1/3rd each to the buyer's state and the seller's state and 1/6th to each municipality respectively with it defaulting to the state if it is unincorporated. It would even actually fit in interstate commerce domain nicely.