We are trying to build a better infrastructure, where pedestrians enjoy safe and pleasant walk, cyclist enjoy safe and pleasant ride, commuters do commute, etc. In order to achieve that, it's important that the spaces are predictable. If you're in a shared space, you expect a bicycle, if you're in a pedestrian area you shouldn't be on a lookout for fast things. Same goes the other way, if you're on a bike in a shared space, you should expect pedestrians be everywhere and should always be on a lookout, but if you're riding a designated bike road, you should be able to enjoy the ride, not crawling with pedestrian speed dodging around.
If this rule doesn't work, the infrastructure doesn't work. You can't expect people using cycling infrastructure for commute if they can't be sure infrastructure is usable, so they wouldn't, so everyone is riding cars and we're back to square one.