What makes you think that they could confiscate one but not the other? King Franklin confiscated U.S. currency along with the rest of the gold people were holding as a source of value. The fact that he stole 43% of the value of people's gold holdings probably contributed to the lack of capital in places that could contribute to economic growth.
That was only part of the equation. Prior to the confiscation, gold was valued at $20/ounce. Afterward, the peg was moved to $35. Anyone holding paper money who had been compensated at the old rate lost 42% of their wealth over night.