At least at the Marriott hotels I stay at, they do not directly refill dispensers in the room, they swap out the bottles with new sealed bottles. I don't have any inside knowledge but my assumption is that the bottles are sanitised, refilled, and are then available to be added back to a room -- like towels, bed sheets and any other reusable product. I think the sanitary concerns are valid but hotels are pretty unsanitary places in general, even if a soap dispenser has the potential for harbouring bacteria, it's not going to come close to being the worst offender in the bathroom, let alone the whole room. I think we all avoid the decorative blankets placed on top of beds like the plague (because they're famously unwashed) but if I were to stop to think for a moment about housekeeping process (do housekeepers wash their hands thoroughly between each room, do they wash them before and after touching fresh towels?) I would never stay at a hotel again.

The links you've shared are to discussions about commercial hand soap dispensers which are a very different proposition. A commercial hand soap dispenser will be used by dozens or even hundreds of people and is constantly topped up which is where the risk of contamination comes in. I also think there's a similar risk with bars of soap because a bar of soap will be placed in one of the small dishes and housekeepers will (in my experience at least) clean that dish.

I guess the ideal is for hotels to include both liquid soap and bars of soap for guests to choose between.