I see these every time a bicycle path intersects with any kind of car or train infrastructure. It just seems really patronizing, as if we're not capable of slowing down without getting off the bike; it also shifts responsibility onto the cyclists if there is a crash based on something arbitrary (mounted vs dismounted).
I've only seen these in my area when you are entering very busy pedestrian areas. It is basically saying that you should expect to be moving slow enough that balancing on your bike will be more hassle than it is worth, so you should just dismount. And if you try to go through at full speed, you're being a dick