Because the rest of the vendors weren't doing what the Board of Equalization required them to do. All the other vendors were in violation of state tax law. It's all about WHERE the sale takes place/physical presence.
Now if they collected the sales tax and didn't forward it to the state agency... Seen that happen more than once.
There was an uproar at MATS a year or two ago with the NV tax agency there wanting/requiring vendors to register and pay taxes on sales.