There are three main reasons people don't DPS as a healer and two of them are totally acceptable:

1) Healer is new and nervous about the giant burst of damage that come in and doesn't want to not be able to cast a cure when they need. This is acceptable.
2) Tanks are undergeared and overpulling and even experienced healers have trouble keeping them alive. This is also acceptable.
3) Lazy, "I am a healer and shouldn't have to do anything else" mentality. This is unacceptable but frequent.