Rice grown in Louisiana and Arkansas (i.e., most rice grown in the US) is indeed high in arsenic.
My guess is that the protein powders are made from brown rice or even rice bran and rice germ whereas most rice in the US is eaten as white rice (i.e., rice without bran or germ). I'm guessing that makes a difference.
Also, if you cook rice as if it were pasta (i.e., in much more water than needed, which is then discarded at the end of cooking) as recommended by the USDA, the amount of arsenic is cut in half. The manufacturers of protein powders probably don't bother to process the rice this way (whereas I do when I cook rice).
It sounds like there should be warnings on the raw elements of protein powders, not just the powders, which makes me think (adjusts tinfoil hat to filter proper wavelengths) that the issue raised by the original article is more about FUD and selling their version of protein powder.
Maybe, maybe not: there is not a lot of protein in rice, and it might be that whatever process is used to extract the protein from the rice also extracts most of the arsenic and that the process of making white rice (again, the kind most Americans eat) gets rid of most of the arsenic, making rice-protein powder much higher in arsenic than the same amount of calories of white rice.
My guess is that the protein powders are made from brown rice or even rice bran and rice germ whereas most rice in the US is eaten as white rice (i.e., rice without bran or germ). I'm guessing that makes a difference.
Also, if you cook rice as if it were pasta (i.e., in much more water than needed, which is then discarded at the end of cooking) as recommended by the USDA, the amount of arsenic is cut in half. The manufacturers of protein powders probably don't bother to process the rice this way (whereas I do when I cook rice).