Is it must to cut doberman's tail????

You do not have to cut the tail of the doberman, it is a person choice that is made with each owner. The tails do grow kind of long and thin with this breed and some people feel the tails need to be cut usually to prevent injury later on in life. Personally I wouldn't want to do that to an animal unless it was medically necessary regardless of breed.

Wouldn't that be considered animal cruelty? Where did you hear you should do that?

I've had dobermans all of my life. You are not obligated to cut their tails. Most are done as puppies so when you buy a dobie that's 8+ weeks old it's already been done. The last litter I bred was over 10 years ago. I did have the pups tails docked but didn't do their ears. At the time I felt it was the way a dobie should look and that it wasn't so very uncomfortable/painful to the pups. If I were still breeding I don't think I would do it now. If the dog is intended to be a pet, docking the tail isn't justified imo and why cause a pup any pain.