There is no law that says you can't switch jobs whenever you want. We literally do have the freedom to do that within the framework of capitalism and the laws that govern the citizens of the USA. The reality of the situation is of course that employers generally don't like that, but employers are not the government and they don't own us. We still have our freedom to choose to pursue whatever we want for employment. These are generally good features of capitalist democracy - it's also good that employers are free not to choose unreliable candidates.