I think the purpose of the cage is pretty clear. It's deployed to prevent drivers from inadvertently ramming the pipe with their front or rear car bumpers. However, think of all the "attacks" for which it is completely unsuited. Here are the first five I could imagine.
- Defacement, like painting obscenities on the pipe
- Cutting the pipe with a saw
- Melting the pipe with a flame
- Cracking the pipe with a hammer
- Stealing water by creating a hole and tube to fill a container
So what if any of these attacks were to happen? Detection and response are my first answers. There's likely a camera somewhere that could see me, my car, and the pipe. Cameras or bystanders are likely to record some detail that would cause the intruder to be identified and later apprehended. Other people in the parking garage are likely to tell someone in authority, or better still, take video or a photo of the intruder in action and then provide that to someone in authority.
So, we can all laugh at the metal cage around this pipe, but it's probably doing just what it needs to do, given the amount of resources available for "defense" and the detection and response "controls" available.
If the defensive posture changed, it would probably not be the result of a security person imagining different attack models against plastic pipes. In other words, it wouldn't be only "decide -> act". Rather, changes would be prompted by observed attacks against real infrastructure. We'd have the full "observe -> orient -> decide -> act" OODA loop. For example, some joker would be seen cutting the pipe using a saw, so patrols and cameras would be enhanced, and possibly wire mesh or plating would be added to the cage to slow down the attacker in time for responders to arrive.