First, the original poster made this post in 2010! Things have changed in the last 3 years.
Second, he didn't say he booked a Future Cruise while on a cruise or that he purchased a Cruise Reward. He said, "When booking my NCL cruise on the Spirit I was told I would receive a $100.00 on on board credit for booking the cruise."
But this is one of the major reasons why you should always book with an agent!! The original poster has not been around except to complain in one of only two posts ever made in this forum, which tells me alot to begin with.
If he was required to go the additional step to get the onboard credit, which I don't agree with, and he didn't go the additional step, then that's no one's fault except his. Things change all the time and just because something happened one way the first time does not mean it will happen the same way all the time. If he booked it directly with the cruise line, then he's left to his own devices and can't blame the cruise line. Had he booked with an agent in the first place, they would have made sure he know all the nuances of the particular requirements associated with getting the onboard credit.
Plus, had he booked with an agent and had any problems, it's the agent's responsibility to handle it so the passenger doesn't have to worry about it. But to complain about the cruise line for something the passenger didn't do is like complaining about the car company when the owner left their doors unlocked and their camera was stolen.