Online retailers charge tax when they have an actual presence in your state.
Simple rule - if they have an address in your state, you got tax.
Since Dell has their random kiosks in malls all over the place, they have an actual presence in many states, and thus charge tax.
FYI, Newegg has warehouses in at least CA, NJ, TN, and those states have to pay tax.
Also note that tax is determined by the shipping address, not location you ordered from.