It was a surprise to me when I first read it about 10 years ago too. I'm having trouble finding the source for the (potential) bacteria breeding problem. But, I did find an EPA document warning not to do it for another reason:

https://www.epa.gov/sites/default/files/2015-09/documents/20...

I’ve heard: Higher levels of dissolved minerals, which might be harmful, and a low chance of being a breeding ground for brain-eating microbes, which will be harmful.

I think it’s specifically Legionnaire’s disease / legionella. Hot water tanks should periodically be heated over 50C to kill off the bacteria. See e.g. https://www.hse.gov.uk/legionnaires/hot-and-cold-water-stora...