Society has a right to regulate the behaviors of individuals where such behaviors affect other non-consenting members of society. Murder and other crimes against the person are the most obvious examples of such behavior.
Yes, any such regulation (even for murder) is partly dictatorial. It's generally considered to be acceptable, though.
The effect of Sunday trading on other members of society is nebulous and difficult to determine, but from my experience it does exist. So it is (arguably) legitimate for society, through the instrument of government, to regulate this behavior.
Personally, I'm quite happy to have shops open on Sunday. I just feel moved to argue against the knee-jerk "if it's government regulation it must be bad" reaction that many Americans seem to have