They are COLLECTING sales taxes, on behalf of your state. They are not "CHARGING" sales taxes. GS, EB, TRU, Amazon, makes nothing off of sales taxes [I think they actually are credited a very small percentage to cover compliance costs, which in today's computerized world should be negligible]. Tru-via-Amazon collects tax, because you're actually buying from TRU, using Amazon's interface.
The basic concept is that if a company 'does business' in a state, they have to collect sales tax for that state. E-stores that have B&M in a state obviously 'do business' in that state. EBGames and EBGames.com used to be two separate operating companies, but combined under one brand, which is why they now have to collect sales tax. Places like Gamefly and Netflix and Blockbuster Online have been deemed to have a 'presence' in most states because they send and receive products to/from that state. To avoid running afoul of the Byzantine mass of tax laws, many etailers collect tax if there's any question at all.
And most states' tax forms now have an 'Estimated online purchases' field on them so they can make some money off the money flowing out of their state.
Bottom line, if you are disappointed at an e-tailers collecting of tax, don't get mad at them, get mad at your state and let your politicians know.
The more important issue is how politicians of all parties and at all levels think *they* have more of a right to your money than *you* do, that they will spend it 'better', and see people buying stuff online and not paying sales tax as a personal affront, and try to get their cut of that money, rather than saying, "hey, you know what, we are already taking 50% of everybody's money". Was it Bloomberg/NYC who was going to start arresting people for buying cigarettes online, when the citizens decided 6 bucks in taxes per pack was a little high?