Singapore exists at the behest of the United States. Its sovereign wealth fund is heavily invested in the United States and its geopolitical precariousness on the straits of malacca is predicated upon the implicit protection of the United States Navy.
It's a political choice to let millionaires flee there, just as it's a political choice not to tax them at home. A snap of the US's fingers will change Singapore's policy.
Like when Wrigley sent a US Congressman to visit - he snapped his fingers and chewing gum was no longer banned.