Polyethylene terephthalate reaction with tap water
It is probably not a chemical reaction that's the problem, it's that the container should be cleaned before it's refilled, and an unclean container might or will have bacteria.  While you're thinking 'refilled' might be in a matter if minutes, someone labeling the bottle is thinking about it sitting on a shelf with the cap off for months before reuse.  At that point there could be any number of unhealthy contaminants.