As mentioned, if a store, like Apple, has a retail presence in a given state then that company is obliged to charge sales tax and remit to that state's government. If not, such as many or most of the resellers, then they don't have to charge you sales tax, but most states have a Use Tax that obliges you to put that, somehow, on your state tax return and pay it.
Now, I don't know about the other states, but my accountant told me that there is simply no practical way for this state to enforce that collection, and therefore they don't. So, I don't pay that sales tax, and in fact go out of my way to buy Apple stuff from resellers that don't have a retail presence here and don't charge sales tax. Saves me 6.5% on those purchases. Nice chunk of change on the Mac Pro and two 30" ACD's I got from B&H Photo, not to mention at least 5 MacBooks/Pros in the last few years.