It's the possiblity of metal contaminants in the piping and water heater. Depending on the age of the house, lead soldering is a common cause of lead, and even 'lead free' pipes can contain up to 8% lead. Copper also can get into the water supply.
Lots of info on the web about it.
A friend of mine works for a company that builds restaurants. They cannot set up the plumbing so that they cook with hot tap water. Any hot water for cooking must be heated in a 'food grade' water heater (different type of heater than what is used for washing the dishes or for employees to wash their hands, or in the bathrooms) and is very expensive he says.