Well, it's not really a law, but attorney has suggested that CBP normally don't terminate H-1B at border vs TN as H-1B does have a dual intent, so they can't just say you have an "immigration intention" to deny your visa at border.
Yup, after a few visits back home US immigration officers have asked me variations on if I was planning on returning to Canada. Regardless of my feelings or intent, the answer is "yes, I plan on returning", especially when a guard is explicitly trying to catch you with intent to immigrate.
And that's definitely an advantage over TN.