The reason you would not want to have the garden tap on soft water is because it is a waste of salt and unnecessary wear and tear on the water softener. Some people think it is because you wouldn’t want to use salty water on the garden. Well, I disagree! I lived by the beach in Selsey, West Sussex and the sea spray would come into my garden and none of my plants or garden ever suffered as a result.
My next point is that salt is not added to the water in order to soften it. The salt is only used to regenerate the water softener. Find out how a water softener works here.
If it has not been possible to keep the garden tap on hard water, you can overcome this problem by bypassing the softener just remember to turn it back on!