Are you saying arresting a violator of that cigarette law isn't written into the law? That police were acting on their own discretion in enforcing it? Wouldn't that be overreaching the law?
I can't say for certain they were acting within or without the law by arresting violators because I haven't read it. I would hope they weren't taking it in their own hands to decide how it should be enforced.
More or less. Selling single cigarettes violates one or more laws. Each law enforcement agency determines which violations of the law they will arrest for and which they will not.
A law enforcement agency's authority will be spelled out in the legislation, typically referred to as enabling legislation, that grants that authority. This language usually reads something along the lines of "officers have the authority to make arrests, serve warrants, and carry firearms to enforce XXX laws." In other words it's fairly general.
The agency, sometimes under other political influences, then determines how they will handle violations (citation, vs, arrest, vs. charging documents) etc.
From the anecdotal information out there, it would appear the agency in question had dictated that their officers were to arrest for the above named offense rather than cite or pursue correction via other means.