I'm not sure why people keep seeing it as immoral.

Is it immoral for a crafter to ask for compensation for his time even if you provide mats? Selling runs is no different from this, you're paying for someone's time and effort.

Selling runs does not mean people will stop helping other players, just like selling crafter services does not mean people will stop helping others with free crafts or melds, you're paying that player a premium to stop doing whatever he's doing and help you with a content clear where you don't even have to remain alive.