I don't think this is 'darwinism at work.' Where engineered controls are inadequate or impractical, it is incumbent on manufacturers to provide sufficient training and placard reminders to prevent users' injury or death. The bottle I have has no warning of what kind of water to use, and the sachets of salt do say what kind of water but it is in small font and they don't caution the user NOT to use tap water. In my case the instructions were lost long ago, so I can't say anything about them.
In my opinion it is far from obvious to a casual user that the use of otherwise safe-to-consume tap water is in this case a potential life hazard. Warnings of such import should be LARGE and in your face, akin to the black box warnings that are the very first things you read on drugs which have a small likelihood but significant risk.
In the example you provided, a single sentence in the middle of the paragraph tells the user not to use tap water, but doesn't bother to mention the risk. They repeat a similar warning without mentioning the risk. I don't know what is the standard for negligence but this certainly seems to me that they aren't providing enough information for a user to be informed.
Finally, your math confuses me. Perhaps your vessel is smaller than mine; mine holds 8 oz. so if used twice a day a gallon will last 8 days, if I don't overfill or spill. Not multiple weeks. Sorry, I can't help myself from the nitpick.
But seriously, the point is not about the cost, it's about Neilmed's failure to provide users with adequate information about the potential risks of failing to follow their simple directions correctly. I think they have not done their job.