I enjoy a good discussion
You are absolutely correct. It is the (state) law! There are 2 potential scenarios that would change this.
1) All the states get together and agree to charge sales tax to other states residents. I don't know that this is likely because the different tax rates makes this 'unfair' for some residents.
2) A federal law is enacted against mail order-type businesses that specifically addresses interstate commerce. The problem I see here is that sales tax typically benefits the state. But if you're talking about a federal tax, then who is benefitting from it?! The states won't likely go along with it if they aren't putting $$$ in their pockets.
As far as your question about what exempts online stores from collecting tax... well, nothing really. If a customer of ours is in PA and purchases something from us then we charge him sales tax. Because as you pointed out, in the state of Pennsylvania, we are required by law to collect it.
However, if the customer is out of state, I am not obligated to collect sales tax (and of course I will not voluntarily do this).
I also got involved with Paintball because I love the sport. However, it has to make enough money to feed my family. For this reason, when things are slow at the store, I blame myself and look at new ways we can market and expand. Online does very well for us, but we realize that to increase our market share we will need to address our local market as well.