One of the issues with forcing a company like Amazon to collect sales tax in all 50 states is that states have weird sales tax laws. For example, in Iowa the sales tax is different depending on the county. So a customer who orders a part in one county has to have X amount of sales tax collected from him. A customer who lives in another county, might have a different. This is hard enough to do in Iowa, but doing it in all 50 states, plus maintaining these tax tables (I am assuming every state is different in regards to taxing shipping costs, etc).
I am not against taxes, but the more hurdles we put in front of commerce, the more we will hurt entrepreneurship.