Why knock it down, when you can just build the wall on the other side of the patrol road, and have two barriers?
Those bollards deter a lot of people (women, children, elderly - kind of a fitness test for manual laborers sneaking into the country). Illegal crossings typically decline more than 90% where they are installed. They are so solid and durable that can even stop a speeding truck from ramming through.
Most of all, Border Patrol prefers bollards to solid wall, so they can constantly observe what is going on, on the other side - it keeps people away from the barrier, prevents them from working undetected, and alerts the defenders earlier, to respond more quickly. Officers can also immediately shoot through the bollards, attacked (which the guys on the South side know).
Bollards also make great hangers for Concertina wire, which is even more of a problem for illegals when they are 18 feet up in the air.
Marines festooning some border bollards witheir own special Christmas garland:
