I’m assuming the union is referring to part 121.391 if it noted that “it’s against the regs for passengers to be in the cabin unless there are also flight attendants with them.” But it also seems to be talking out both sides of it’s face by also saying … “Leaving passengers on the aircraft unattended is a significant safety and security concern.”
It is very good that a labour union is educating/reinforcing its members on doing their job well.
As for security, there’s an opportunity for theft or tampering.
Recent case(s) of unticketed passenger hiding in lavatories highlights risks.
BTW, they can leave pilots on board. 
A personal risk to cleaning staff, who could be assaulted.
As for theft, first aid kits, fire extinguishers, …
Back before deregulation I once flew on a United 747 from Chicago to Seattle as one of 17 passengers. After we were up and cruising the flight attendants set up a self service bar in the back of the main cabin and then they all disappeared upstairs for the rest of the flight until landing. I had fun checking out seat positions from the most forward where you almost felt in the cockpit to the most rearward where you could see the twisting of the fuselage when the plane banked. The tail clearly banked a split second after the wings.
If someone is “sleeping” under the seats in economy they are either a contortionist, been knocked out by someone getting their bags down from the overhead compartments or, most likely, they have been trampled to death by passengers seated around them that are trying be the first ones off the airplane.
Scraping the barrel for content I see.
From Russ:
Got me. It happens. My rule when it does is to find something funny and light. I think this filled the bill.
Don’t laugh at “sleeping under the seats”.
While in the USAF, (1966) my wife, young son, and I were flying back to Tachikawa Japan from Travis AFB in a Canadian CL-44. Only passengers, no cargo.
Around 0200 local time, my wife woke me and asked where our son was. Huh? He couldn’t walk yet, but somehow he slipped out of his baby seat, and was GONE. Was I dreaming this?
I looked all around us, even under our seats ---- no son! I went to the back of the cabin and asked a Flight Attendant to help me find my son. She looked a bit perplexed but as she looked at other seats, I crawled on the floor looking under EVERY seat, hoping someone didn’t raise an alarm about some PTSD weirdo looking at everyone’s legs under the seats.
Alas - I finally found him asleep on the floor, about 12 rows forward of our seats on the Port aisle under the second seat in.
I’m glad to this day we weren’t on a B-747 or DC-10.
Considering the kind of news we’ve been having over the past month or so; a lack of ‘interesting’ stories isn’t that bad.