Let's assume for a moment that it is only fair that online retailers pay sales tax too. Put aside the fact that online sales of anything physical requires shipping. And shipping involves many other taxes, such as fuel taxes.
Now the problem that arises is not so much the tax in itself (that will just serve to raise consumer costs approx. 6%, but benefit state coffers). The real problem is in administration of that tax. Not only will it cost small retailers more to sell on the internet, it will be effectively IMPOSSIBLE to do it on one's own. It will simply be too burdensome to manage all the sales brackets and filings. So prices will rise even more cover the new administrative overhead. The inevitable result will be the erosion of small online retail companies and the loss of more high-tech jobs.
What's really sad about this, is that there is a simple solution to at least preventing the admin overhead cost.... have the shipping companies levy the tax. They are already fully equipped to levy charges per zip-code. There are only a handful of companies involved, and they are all large businesses. So the new overhead for them would be negligible.