It's not USD purchase specifically, it's exchanging your RMB for any foreign currency that's restricted. The Communist Party keeps people's money hostage and the reason is very clear: The rich obviously do not trust that they're safe. There is no rule of law, you can be arbitrarily locked up and have your savings taken from you at any time. All it takes for example is a change in leaders, as the Xi purge has shown. So they take their money abroad. To Canada, to the US, the UK and elsewhere.
The bill is probably needed but this is a bad look for the US whichever way you view it. I could go into long details but in short the situation we have now with increasing trade restrictions, stock delistings and so forth is the culmination of catastrophically failed policy towards the Chinese regime. The US and other developed countries were completely wrong and naive in the way they dealt with China. In fairness, in the 80s and 90s no one could have foreseen the current development and it was a reasonable assumption that with economic growth the country would liberalize. Even under Hu/Wen, I can see how people could convince themselves into believing increasing economic ties would make things better. Although already back in the late 2000s it was clear that the promise to open up the markets was a lie that never happened. Foreign ownership continued to be restricted, companies were forced into joint ventures (aka "thanks for the blueprints, suckers"), the Yuan control actually got tighter...
Western countries plus Japan, Korea and Taiwan made the Communist Party rich and powerful. Without all that foreign investment and technology transfer Mainland China would still be a broken backwater. Corporations in the developed world had gold glittering in their eyes when they looked at China and it's massive population. The Chinese regime found that one fatal flaw in free market capitalist systems: While economically they work, there is no incentive for the corporate leadership to do what's best for the country. Those who spearheaded the outsourcing of jobs and factories profited massively. As did the shareholders initially. It only came at the cost of the future of all the less wealthy people in the respective countries. Plus now they have a nuclear power pouring more and more billions into their military, preparing for war, while running concentration camps and allying with Russia and Iran.