It's a good question. Maybe something with interstate commerce laws?
That loophole got closed once inter-state data sharing became possible and Oregon merchants were required to start collecting those out-of-state taxes at the point of sale.
Avoiding taxes. It's different. It was always perfectly legal to travel to another state to buy something expensive and bring it back home. No crimes were committed.
It was a loophole that you could buy in Oregon specifically to avoid $1,000s in sales taxes.
It was legal to do that. If it was purchased out of state with the intent of bringing it back home, then (assuming the home state was California) California use taxes were always owed on it. Other states with sales taxes also tend to have similarly-structured use taxes with rates similar to the sales tax rates.
They were legally avoiding sales taxes, but also illegally evading use taxes, and, moreover, there is very little reason for the former if you aren't also doing the latter, unless you just have some moral objection to your taxes being taken at the point of sale and the paperwork and remittance to the government being done by the retailer instead of being a burden you deal with yourself.