Completing a nightfall is earning as well.

Kind of. Most activities in Destiny have stated rewards. You get Crucible Marks and Vanguard Gear from Crucible. You get Vanguard Marks and Vanguard gear from Strikes. You get Reputation from Patrols. Etc, etc. During all of these activities you have a chance to be rewarded with additional stuff. But that's all it is, a chance. And it's addition to the rewards that you earn. Think of it this way. At the end of your work month you get a paycheck. That's the reason you go to work. You work for an agreed upon amount, and that's what you get at the end of the month. You earned it. If, while at work, you find a $20 bill on the floor, that doesn't mean that you earned that $20. You were just lucky. You're right that Nightfall is a little different. Nightfall has unstated rewards, and those rewards can include Exotic gear.So you could say that running Nightfalls in the hope of getting Exotic Gear is earning it. But thats like saying that showing up to work every day in the hope that the building collapses on you is earning whatever insurance settlement you get out of it. Even if you don't agree that Nightfall rewards aren't earned, it's still ridiculous to claim that the people buying stuff from Xur aren't earning their Strange Coins (and i have no idea if you are claiming that, but that's certainly the popular opinion around here).