Because they are ignorant and cruel.
The ONLY reasons you can 'Dock' a dogs tail in the UK are for either Medical reasons, or if the dog is a working animal, and there is a risk of it being injured if it has a tail.
Any other reason is illegal, and is as already stated Ignorant and cruel!
Any Schnauzer Mini, Standard or Giant, look beautiful with a full tail, which is very expressive!