I'd think German traffic engineers could come up with a better solution, yet these are everywhere:
The bike paths themselves are desire paths for cars and motorcycles.
To block them off, they put up barriers, but those also block cargo bikes, wheelchairs and strollers, create a collision hazard, and force all cyclists to stop and dismount for no reason.
Of course, they'll be bypassed wherever possible – naturally by cars and motorcycles too, if there's enough room.
At least for the dams I can answer that. The posts, if removed, are a hazard when there is a flood. You’d need extra man power/time to collect excess posts in a moment when these factors are critical in fighting against flooding. Yes, it’s not the standard to drive atop the dam, but before starting to fortify it there needs to be a measurement of how well it’s doing where.