Why can't you cook frozen chicken in a slow-cooker (crockpot)?

[cmhi]

According to the usda website:

"Do not cook frozen chicken in a slow cooker or in the microwave; thaw it before cooking). However, chicken can be cooked from the frozen state in the oven or on the stove. The cooking time may be about 50% longer."

Why can you cook frozen chicken in an oven but not in a slow-cooker?

[Listherix]

It would take way too long to thaw in the slow cooker, so it might promote salmonella poisoning if not fully cooked.