All of the "backflow preventers" I've seen have been vacuum breakers, required to prevent chemicals from being sucked into the supply lines if the water pressure fails, which would create a siphon effect. A vacuum breaker will open up when pressure drops, letting air be sucked into the system instead of chemicals. (You see these things on darkroom sinks, they look like a flat-bell-kinda thing at the top of a pipe.)
A check valve, on the other hand, is a one-way valve, i.e., a spring-loaded ball. Water can go one way because it pushes the ball into the spring, away from the valve seat. It can't go the other way, because it pushes the ball into the valve seat.
"Also, how would you prevent terrorists from disabling them?"
With the former, you don't. With the latter, you bury them outside the building, between the water mains and the distribution pipe to the building.
IMO the article paints an overly optimistic picture of the threat. What? Yeah, overly optimistic.
That stuff about "a sudden drop in water pressure in a targeted neighborhood as terrorists stopped the flow of water into a home or business" is IMO feel-good nonsense. If the bad guys hooked up a high pressure pump to a water faucet, opened the faucet, and started the pump injecting concentrated toxin into the pipes at a low rate of volume, I dunno, off the top of my head, a gallon or two a minute, they'd be able to effect a considerable amount of havoc with zero detectibility. I mean, hell, there'd be less pressure change from that than there'd be from someone turning on (or off) the faucet to wash his hands.
And where they got that stuff about a vacuum cleaner, I have no idea.
The problem, in a nutshell, is that our infrastructure -- from top to bottom -- was not designed for survivability in hell. It was designed to perform OK in a normal country, populated with normal people. You don't put a half-million dollars worth of bomb and radiation hardening around a ramshackle outhouse in the backyard. You put a hook-latch on the wooden door. IOW, you build the "security" measures to be commensurate with the anticipated level of threat.
We're facing the kind of nightmare that can only occur when a free society is attacked by a group of sociopaths. And the solution is to either retrofit an ever-increasing amount of safety measures (i.e., checkvalves on every supply line to every house and business in the country -- and mind you, that will only address the "poison into the pipes" problem, all other weak points will remain unprotected), or, to deal with the perpetrators of the crimes, and deal with them quickly, and mercilessly.
It's either that, or we let them win.
We cannot make our country safe against attack other than by killing all those who would attack it. Sad, but true.
And something the attackers should have thought seriously about before starting in. Although, from their warped perspective our non-response to their previous, less-efficient attacks essentially were a form of entrapment. We suckered them into this situation, and now they're going to die for it!