In our trips in the western USA, we have found many locations with nonpotable water in the restrooms and for other washing/watering purposes (e.g., flushing trailer septic systems). Signs have explicitly warned that the water is nonpotable. Is it actually somehow contaminated or just not certified as safe for consumption--who knows? The only safe course is not to drink it. Pollutants could be biological, mineral, or chemical. If it's chemical, purification is not likely to be practicable for a hiker. It's quite possible that some businesses may have decided it is more cost effective or otherwise practical to limit themselves to drinking from bottled water. Presumably businesses which prepare food on site will have potable water.

Incidentally, even if the water is considered potable, you still may not want to drink it if you have alternate water/liquid sources. For example, on a recent camping trip in Death Valley, my younger son & I drank water from drinking fountains. Neither my wife or older son, who did not drink the local water had any complaints. Obviously the water was potable. On the other hand, it was sufficiently mineral laden, that we had many urgent trips to the restrooms. In short the water was not really poisonous, but it certainly wasn't desireable either. We were happy to have brought our own water supply, especially since we neglected to bring a filter.

John