animal rights activist are against the use of silk.why?

During the rearing of silk,the caterpillars starts secreting the silk fibre around itself in the pupa stage. This caterpillar further secretes fibre & itself get covered under the thread. This is the cocoon stage. Cocoon has to break this fibre to come into the world but culturists need fibres to make silk fabric so they place it in hot water which kills the cocoon and culturist get fibres. Because of this reason, animal rights activist are not in the favour of rearing of silk.