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.
Oregon merchants are not required to collect sales tax for any other jurisdictions outside of Oregon. And they don’t, any non Oregonian can go to any merchant in Oregon right now, and you will be charged the same as any other customer who lives in Oregon.
Also, it was never a loophole to buy things in Oregon to evade sales tax. All states with sales tax require their residents to remit use tax for any items brought into the state to make up the difference for any sales tax that would have been paid had it been purchased in their home state.