The US is outraged: why would a Russian polling organization be required to register as a foreign agent?

"In a letter, prosecutors said Levada could not continue to release its work without identifying itself as “a foreign agent,” as required under a new law...The prosecutor’s letter, dated Wednesday, details a series of grants the center received from the MacArthur Foundation, the Ford Foundation and the Open Society Institute Foundation, amounting to around $800,000 over a three-year period. Mr. Gudkov said foreign sources accounted for no more than 3 percent of the center’s funding."  Correct me if I am wrong, is it different in the US? If a US research group were to receive Russian funding in any amount, would it not be required to register as a foreign agent?