I'm sure they didn't charge sales tax because they wanted to, it doesn't benefit them and, by the way, it's THEIR sales.
New legislation is requiring online retailers to charge state sales tax even if they do not have a physical presence in your state. If they do over a certain $$ value of sales in a particular state then they must collect sales tax and distribute it to that state. It is being phased in a little at a time starting either late last year or early this year.