I have a few associated accounts. When I go on another Stack Exchange site, I'm tempted to associate it for the 100 point bonus, even if I'm only reading and not posting. Is this greedy? Would it be unethical to associate with sites I don't even read for the 100 point bonus?
You're supposed to associate your accounts. That's to help the system much more than being some sort of reputation-grabbing, free give-away for the greedy.
Remember, reputation is a rough measurement of your experience with the system. The +100-rep association bonus is to give you a quick boost for the experience you have already EARNED on other systems. That bit of reputation empowers you with some of the most basic (and sorely needed) resources needed to be spread across the sites.
There is absolutely nothing unethical or greedy about it.
By getting 200 points on site A you've demonstrated you know how to ask and answer questions, vote, comment and otherwise participate in the site.
By giving you a 100 point bonus the system is acknowledging that level of expertise and allowing you to do it from "day one" on site B.
By having more people who can do these things and have demonstrated they know the way Stack Exchange operates site B will be a better place and will (hopefully) be more likely to succeed.