Yes, there is no specific legal precedence for this 6 months rule. It should not be a cause for concern for switching employers. There could be a number of reason one would change a employer. Just be ready with one of those valid reasons and then you are good. People change employers all the time. If you're bound to a employer then it would be sort of a slave market. I don't think that is what USCIS expect when they approve H-1B or green card etc. H-1B can be transferred and green card allows you to work for any employer in the United States.